Co-reporter:Yea Ram Lee, Daehwan Park, Sang Koo Choi, Miju Kim, Heung Soo Baek, Jin Nam, Chan Bok Chung, Chinedum O. Osuji, and Jin Woong Kim
ACS Applied Materials & Interfaces September 13, 2017 Volume 9(Issue 36) pp:31095-31095
Publication Date(Web):August 17, 2017
DOI:10.1021/acsami.7b08783
Cellulose fibrils, unique plant-derived semicrystalline nanomaterials with exceptional mechanical properties, have significant potential for rheology modification of complex fluids due to their ability to form a physically associated semiflexible fibrillary network. Here, we report new associative cellulose nanocrystals (ACNCs) with stress-responsive rheological behaviors in an aqueous solution. The surface-mediated living radical polymerization was employed to graft poly(stearyl methacrylate-co-2-methacryloxyethyl phosphorylcholine) brushes onto the nanofibrils, and then 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation was conducted to produce nanoscale ACNCs in the aqueous solution. The ACNCs displayed interfibril association driven by the hydrophobic interaction that resulted in the formation of a nanofibrillar crystalline gel phase. We observed that the viscosity of the ACNC fluid showed reversible shear thinning and temperature-induced thickening in response to applied shear stress and thermal shock. Moreover, thanks to generation of a mechanically robust nanofibrillar crystalline gel network, the ACNC suspension showed extraordinary stability to changes in salinity and pH. These results highlighted that the interfibril hydrophobic association of ACNCs was vital and played an essential role in regulation of stimuli-responsive sol–gel transitions.Keywords: associative cellulose nanocrystals; hydrophobic interaction; nanofibrillar crystallinity; smart nanofluids; sol−gel transition;
Co-reporter:Xunda Feng, Kohsuke Kawabata, Dylan M. Whang, and Chinedum O. Osuji
Langmuir October 10, 2017 Volume 33(Issue 40) pp:10690-10690
Publication Date(Web):September 8, 2017
DOI:10.1021/acs.langmuir.7b02467
We present a strategy for robustly cross-linking self-assembled lamellar mesophases made from plant-derived materials to generate polymer nanosheets decorated with a high density of functional groups. We formulate a supramoleclar complex by hydrogen-bonding conjugated linoleic acid moieties to a structure-directing tribasic aromatic core. The resulting constructs self-assemble into a thermotropic lamellar mesophase. Photo-cross-linking the mesophase with the aid of an acrylate cross-linker yields a polymeric material with high-fidelity retention of the lamellar mesophase structure. Transmission electron microscopy images demonstrate the preservation of the large area, highly ordered layered nanostructures in the polymer. Subsequent extraction of the tribasic core and neutralization of the carboxyl groups by NaOH result in exfoliation of polymer nanosheets with a uniform thickness of ∼3 nm. The nanosheets have a large specific area of ∼800 m2/g, are decorated by negatively charged carboxylate groups at a density of 4 nm–2, and exhibit the ability to readily adsorb positively charged colloidal particles. The strategy as presented combines supramolecular self-assembly with the use of renewable or sustainably derived materials in a scalable manner. The resulting nanosheets have potential for use as adsorbents and, with further development, rheology modifiers.
Co-reporter:Gilad Kaufman, Wei Liu, Danielle M. Williams, Youngwoo Choo, Manesh Gopinadhan, Niveditha Samudrala, Raphael Sarfati, Elsa C. Y. Yan, Lynne Regan, and Chinedum O. Osuji
Langmuir November 28, 2017 Volume 33(Issue 47) pp:13590-13590
Publication Date(Web):November 2, 2017
DOI:10.1021/acs.langmuir.7b03226
Protein adsorption and assembly at interfaces provide a potentially versatile route to create useful constructs for fluid compartmentalization. In this context, we consider the interfacial assembly of a bacterial biofilm protein, BslA, at air–water and oil–water interfaces. Densely packed, high modulus monolayers form at air–water interfaces, leading to the formation of flattened sessile water drops. BslA forms elastic sheets at oil–water interfaces, leading to the production of stable monodisperse oil-in-water microcapsules. By contrast, water-in-oil microcapsules are unstable but display arrested rather than full coalescence on contact. The disparity in stability likely originates from a low areal density of BslA hydrophobic caps on the exterior surface of water-in-oil microcapsules, relative to the inverse case. In direct analogy with small molecule surfactants, the lack of stability of individual water-in-oil microcapsules is consistent with the large value of the hydrophilic–lipophilic balance (HLB number) calculated based on the BslA crystal structure. The occurrence of arrested coalescence indicates that the surface activity of BslA is similar to that of colloidal particles that produce Pickering emulsions, with the stability of partially coalesced structures ensured by interfacial jamming. Micropipette aspiration and flow in tapered capillaries experiments reveal intriguing reversible and nonreversible modes of mechanical deformation, respectively. The mechanical robustness of the microcapsules and the ability to engineer their shape and to design highly specific binding responses through protein engineering suggest that these microcapsules may be useful for biomedical applications.
Co-reporter:Yekaterina Rokhlenko, Paweł W. Majewski, Steven R. Larson, Padma Gopalan, Kevin G. Yager, and Chinedum O. Osuji
ACS Macro Letters April 18, 2017 Volume 6(Issue 4) pp:404-404
Publication Date(Web):March 28, 2017
DOI:10.1021/acsmacrolett.7b00036
Recent experiments have highlighted the intrinsic magnetic anisotropy in coil–coil diblock copolymers, specifically in poly(styrene-block-4-vinylpyridine) (PS-b-P4VP), that enables magnetic field alignment at field strengths of a few tesla. We consider here the alignment response of two low molecular weight (MW) lamallae-forming PS-b-P4VP systems. Cooling across the disorder–order transition temperature (Todt) results in strong alignment for the higher MW sample (5.5K), whereas little alignment is discernible for the lower MW system (3.6K). This disparity under otherwise identical conditions of field strength and cooling rate suggests that different average grain sizes are produced during slow cooling of these materials, with larger grains formed in the higher MW material. Blending the block copolymers results in homogeneous samples which display Todt, d-spacings, and grain sizes that are intermediate between the two neat diblocks. Similarly, the alignment quality displays a smooth variation with the concentration of the higher MW diblock in the blends, and the size of grains likewise interpolates between limits set by the neat diblocks, with a factor of 3.5× difference in the grain size observed in high vs low MW neat diblocks. These results highlight the importance of grain growth kinetics in dictating the field response in block copolymers and suggests an unconventional route for the manipulation of such kinetics.
Co-reporter:Gilad Kaufman;Shomeek Mukhopadhyay;Yekaterina Rokhlenko;Siamak Nejati;Rostislav Boltyanskiy;Youngwoo Choo;Michael Loewenberg
Soft Matter (2005-Present) 2017 vol. 13(Issue 15) pp:2733-2737
Publication Date(Web):2017/04/12
DOI:10.1039/C7SM00092H
Microcapsules with high mechanical stability and elasticity are desirable in a variety of contexts. We report a single-step method to fabricate such microcapsules by microfluidic interfacial complexation between high stiffness cellulose nanofibrils (CNF) and an oil-soluble cationic random copolymer. Single-capsule compression measurements reveal an elastic modulus of 53 MPa for the CNF-based capsule shell with complete recovery of deformation from strains as large as 19%. We demonstrate the ability to manipulate the shell modulus by the use of polyacrylic acid (PAA) as a binder material, and observe a direct relationship between the shell modulus and the PAA concentration, with moduli as large as 0.5 GPa attained. These results demonstrate that CNF incorporation provides a facile route for producing strong yet flexible microcapsule shells.
Co-reporter:Manesh Gopinadhan;Youngwoo Choo;Lalit H. Mahajan;Dennis Ndaya;Gilad Kaufman;Yekaterina Rokhlenko;Rajeswari M. Kasi
Molecular Systems Design & Engineering (2016-Present) 2017 vol. 2(Issue 5) pp:549-559
Publication Date(Web):2017/12/04
DOI:10.1039/C7ME00070G
Magnetic fields are useful for directing block copolymer (BCP) self-assembly, but to date such a field alignment has required large fields (>5 T) necessitating the use of superconducting magnets. We report an approach that circumvents this limitation by introducing labile reactive mesogens into a liquid crystalline (LC) BCP based on a norbornene backbone with a poly(lactide) minority block that forms hexagonally packed cylinders. The free mesogens co-assemble with the smectic A mesophase of the BCP and enable alignment at fields as low as 0.5 T. The remarkable field response originates from the combined effects of enhanced mobility and decreased segregation strength, and the presence of large micron-scale grains in the system. We demonstrate a robust alignment of mesogen-blended samples using simple permanent magnets. The etching of poly(lactide) yields nanoporous films, while the spatially selective microdomain immobilization by UV-induced crosslinking through a photomask provides a versatile mechanism for creating alignment patterns. We anticipate that the nanoporous materials as generated here may find application in membrane fabrication or BCP lithography, while the ability to spatially pattern alignment is promising for the design of mechanical metamaterials exploiting the shape memory effect of LC elastomers.
Co-reporter:Candice I. Pelligra, Kristof Toth, Hanqiong Hu and Chinedum O. Osuji
Nanoscale 2016 vol. 8(Issue 1) pp:149-156
Publication Date(Web):23 Nov 2015
DOI:10.1039/C5NR07914D
We present a facile method for the synthesis of nanorod arrays over large areas with fine control over the average rod–rod spacing. Block copolymer micelles are used to template solvothermal synthesis of ZnO nanorods by preferentially enabling reactant diffusion through the micelle cores to an underlying seed layer. The distance between nanorod centers is defined by the micelle number density which is in turn controlled by the molecular weight of the block copolymer, and the block copolymer concentration in a templating film. We demonstrate the ability to control the resulting nanorod number density from ∼100 μm−2 down to ∼10 μm−2 with high fidelity. Correspondingly, the distance between nanorod surfaces was varied from ∼60 nm to 230 nm. The method developed here provides a viable approach for rapidly fabricating large-area nanostructured electrodes comprised of nanorod arrays with controlled geometries. The ability to tailor nanorod spacing over a broad range suggests applications in photovoltaics and sensors based on optical resonances can be readily addressed.
Co-reporter:Manesh Gopinadhan, Youngwoo Choo, and Chinedum O. Osuji
ACS Macro Letters 2016 Volume 5(Issue 3) pp:292
Publication Date(Web):February 18, 2016
DOI:10.1021/acsmacrolett.5b00924
We elucidate the roles of the isotropic–nematic (I–N) and nematic–smectic A (N–SmA) transitions in the magnetic field directed self-assembly of a liquid crystalline block copolymer (BCP), using in situ X-ray scattering. Cooling into the nematic from the disordered melt yields poorly ordered and weakly aligned BCP microdomains. Continued cooling into the SmA, however, results in an abrupt increase in BCP orientational order with microdomain alignment tightly coupled to the translational order parameter of the smectic layers. These results underscore the significance of the N–SmA transition in generating highly aligned states under magnetic fields in these hierarchically ordered materials.
Co-reporter:Nawal Quennouz, Sara M. Hashmi, Hong Sung Choi, Jin Woong Kim and Chinedum O. Osuji
Soft Matter 2016 vol. 12(Issue 1) pp:157-164
Publication Date(Web):09 Oct 2015
DOI:10.1039/C5SM01803J
Cellulose nanofibrils (CNFs) present unique opportunities for rheology modification in complex fluids. Here we systematically consider the effect of ionic and non-ionic surfactants on the rheology of dilute CNF suspensions. Neat suspensions are transparent yield-stress fluids which display strong shear thinning and power-law dependence of modulus on concentration, G′ ∼ c2.1. Surfactant addition below a critical mass concentration cc produces an increase in the gel modulus with retention of optical clarity. Larger than critical concentrations induce significant fibril aggregation leading to the loss of suspension stability and optical clarity, and to aggregate sedimentation. The critical concentration was the lowest for a cationic surfactant (DTAB), cc ≈ 0.08%, while suspension stability was retained for non-ionic surfactants (Pluronic F68, TX100) at concentrations up to 8%. The anionic surfactant SDS led to a loss of stability at cc ≈ 1.6% whereas suspension stability was not compromised by anionic SLES up to 8%. Dynamic light scattering data are consistent with a scenario in which gel formation is driven by micelle–nanofibril bridging mediated by associative interactions of ethoxylated surfactant headgroups with the cellulose fibrils. This may explain the strong difference between the properties of SDS and SLES-modified suspensions. These results have implications for the use of CNFs as a rheology modifier in surfactant-containing systems.
Co-reporter:Youngwoo Choo;Hanqiong Hu;Kristof Toth
Journal of Polymer Science Part B: Polymer Physics 2016 Volume 54( Issue 2) pp:247-253
Publication Date(Web):
DOI:10.1002/polb.23913
ABSTRACT
The delivery of sub-micron droplets of dilute polymer solutions to a heated substrate by electrospray atomization enabled precisely controlled and continuous deposition, or growth, of block copolymer thin films. It also provided, in principle, the ability to fabricate heterolattice materials using sequential depositions. This possibility was explored and the morphology of resulting composite films produced by such sequential electrospray deposition (ESD) of lamellar diblock copolymers of poly(styrene-b-4-vinylpyridine) with differing molecular weights was examined. The structure of the heterolattice interface was a strong function of temperature. Sharp interfaces with abrupt changes in the lamellar period were observed at lower deposition temperatures, while higher temperatures produced a smooth variation in the lamellar period from one molecular weight to the next. The ordering kinetics of a secondary high molecular weight layer could be substantially enhanced depending on the molecular weight of the polymer present in the underlying primary layer. These findings were discussed in the context of temperature and molecular weight dependent diffusion dynamics of the polymers in the melt which control the inter-mixing of the layers and therefore the structure of the heterolattice interface. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 247–253
Co-reporter:Xunda Feng, Siamak Nejati, Matthew G. Cowan, Marissa E. Tousley, Brian R. Wiesenauer, Richard D. Noble, Menachem Elimelech, Douglas L. Gin, and Chinedum O. Osuji
ACS Nano 2016 Volume 10(Issue 1) pp:150
Publication Date(Web):December 3, 2015
DOI:10.1021/acsnano.5b06130
Membrane separations are critically important in areas ranging from health care and analytical chemistry to bioprocessing and water purification. An ideal nanoporous membrane would consist of a thin film with physically continuous and vertically aligned nanopores and would display a narrow distribution of pore sizes. However, the current state of the art departs considerably from this ideal and is beset by intrinsic trade-offs between permeability and selectivity. We demonstrate an effective and scalable method to fabricate polymer films with ideal membrane morphologies consisting of submicron thickness films with physically continuous and vertically aligned 1 nm pores. The approach is based on soft confinement to control the orientation of a cross-linkable mesophase in which the pores are produced by self-assembly. The scalability, exceptional ease of fabrication, and potential to create a new class of nanofiltration membranes stand out as compelling aspects.Keywords: membrane separation; soft confinement; thin film;
Co-reporter:Jonathan P. Singer, Manesh Gopinadhan, Zhen Shao, André D. Taylor, Jan Schroers, and Chinedum O. Osuji
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 6) pp:3456
Publication Date(Web):January 31, 2015
DOI:10.1021/am507368g
The use of bulk metallic glass (BMG) for the nanoimprint of high-aspect-ratio (>3) features into functional polymers is investigated. To accomplish this, the most critical aspect is the successful demolding of the imprinted polymer. By fluorosilane functionalization of the BMG surface and optimization of processing temperature, high aspect pore features down to 45 nm diameters are introduced into the surface of two organic photovoltaic systems: poly(3-hexylthiophene-2,5-diyl) (P3HT) and 1:1 mixtures of P3HT with Phenyl-C61-butyric acid methyl ester (PCBM). The crystallinity of P3HT demands higher forming temperatures and pressures that are difficult to obtain with conventional soft nanoimprint lithography molds. The ability to accommodate a wide range of processing conditions and the low cost of fabricating molds with nanometer-scale features point to the large potential of nanotextured BMGs as an economical and scalable imprint material for high-resolution applications.Keywords: bulk heterojunctions; bulk metallic glass; nanoimprint lithography; organic photovoltaics; polymer patterning
Co-reporter:Hanqiong Hu;Youngwoo Choo;Xunda Feng
Macromolecular Rapid Communications 2015 Volume 36( Issue 13) pp:1290-1296
Publication Date(Web):
DOI:10.1002/marc.201500099
Co-reporter:Gilad Kaufman, Siamak Nejati, Raphael Sarfati, Rostislav Boltyanskiy, Michael Loewenberg, Eric R. Dufresne and Chinedum O. Osuji
Soft Matter 2015 vol. 11(Issue 38) pp:7478-7482
Publication Date(Web):07 Jul 2015
DOI:10.1039/C5SM00973A
Composite microcapsules have been aggressively pursued as designed chemical entities for biomedical and other applications. Common preparations rely on multi-step, time consuming processes. Here, we present a single-step approach to fabricate such microcapsules with shells composed of nanoparticle–polyelectrolyte and protein–polyelectrolyte complexes, and demonstrate control of the mechanical and release properties of these constructs. Interfacial polyelectrolyte–nanoparticle and polyelectrolyte–protein complexation across a water–oil droplet interface results in the formation of capsules with shell thicknesses of a few μm. Silica shell microcapsules exhibited a significant plastic response at small deformations, whereas lysozyme incorporated shells displayed a more elastic response. We exploit the plasticity of nanoparticle incorporated shells to produce microcapsules with high aspect ratio protrusions by micropipette aspiration.
Co-reporter:Youngwoo Choo, Lalit H. Mahajan, Manesh Gopinadhan, Dennis Ndaya, Prashant Deshmukh, Rajeswari M. Kasi, and Chinedum O. Osuji
Macromolecules 2015 Volume 48(Issue 22) pp:8315-8322
Publication Date(Web):November 16, 2015
DOI:10.1021/acs.macromol.5b02009
We explore the morphology and phase behavior of a recently introduced architecture of liquid crystalline brushlike block copolymer (LCBBC) as functions of composition and molecular weight. Low-polydispersity materials were prepared by ring-opening metathesis polymerization of n-alkyloxycyanobiphenyl and poly(dl-lactide) (PLA) functionalized norbornene monomers. Well-ordered block copolymer mesophases were observed with transitions from spheres to hexagonally packed cylinders, lamellae, inverse cylinders, and inverse spheres on increasing the weight fraction of the liquid crystalline block, fLC, from 0.15 to 0.85. The microdomain spacing displays a power-law scaling with molecular weight with an exponent of 0.6, L0 ∼ MW0.6. The simple occurrence of microdomains with curved interfaces, spherical and cylindrical, and the sublinear scaling of microdomain spacing with molecular weight set this system clearly apart from bottlebrush block copolymers. We observe a peculiar morphology dependence of the liquid crystal anchoring condition with the cyanobiphenyl mesogens adopting planar anchoring at cylindrical microdomain interfaces while both homeotropic and planar anchoring were displayed at the block interface in lamellar systems.
Co-reporter:Manesh Gopinadhan;Prashant Deshmukh;Youngwoo Choo;Pawel W. Majewski;Olgica Bakajin;Menachem Elimelech;Rajeswari M. Kasi
Advanced Materials 2014 Volume 26( Issue 30) pp:5148-5154
Publication Date(Web):
DOI:10.1002/adma.201401569
Co-reporter:Marissa E. Tousley, Xunda Feng, Menachem Elimelech, and Chinedum O. Osuji
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 22) pp:19710
Publication Date(Web):September 2, 2014
DOI:10.1021/am504730b
Magnetic-field-directed assembly of lyotropic surfactant mesophases provides a scalable approach for the fabrication of aligned nanoporous polymers by templated polymerization. We develop and characterize a lyotropic liquid crystalline system containing hexagonally packed cylindrical micelles of a polymerizable surfactant in a polymerizable solvent. The system exhibits negative magnetic anisotropy, resulting in the degenerate alignment of cylindrical micelles perpendicular to the magnetic field. Sample rotation during field alignment is used to effectively break this degeneracy and enable the production of uniformly well-aligned mesophases. High-fidelity retentions of the hexagonal structure and alignment were successfully achieved in polymer films produced upon UV exposure of the reactive system. The success of this effort provides a route for the fabrication of aligned nanoporous membranes suitable for highly selective separations, sensing, and templated nanomaterial synthesis.Keywords: directed self-assembly; lyotropic mesophase; nanoporous polymers; polymer membranes
Co-reporter:Gilad Kaufman, Rostislav Boltyanskiy, Siamak Nejati, Abdou R. Thiam, Michael Loewenberg, Eric R. Dufresne and Chinedum O. Osuji
Lab on a Chip 2014 vol. 14(Issue 18) pp:3494-3497
Publication Date(Web):09 Jul 2014
DOI:10.1039/C4LC00482E
Common methods for fabrication of polyelectrolyte microcapsules rely on a multi-step process. We propose a single-step approach to generate polyelectrolyte microcapsules with 1–2 μm shells based on polyelectrolyte complexation across a water/oil droplet interface and study the effect of parameters controlling the polyelectrolyte complexation on shell thickness.
Co-reporter:Hanqiong Hu, Manesh Gopinadhan and Chinedum O. Osuji
Soft Matter 2014 vol. 10(Issue 22) pp:3867-3889
Publication Date(Web):21 Mar 2014
DOI:10.1039/C3SM52607K
Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.
Co-reporter:Hanqiong Hu, Jonathan P. Singer, and Chinedum O. Osuji
Macromolecules 2014 Volume 47(Issue 16) pp:5703-5710
Publication Date(Web):August 13, 2014
DOI:10.1021/ma500376n
Electrospray has been recently advanced as a novel approach for the continuous deposition of self-assembled block copolymer thin films. It represents an analogue of physical vapor deposition in which the development of well-ordered microstructures is predicated on relatively rapid relaxation of the polymer compared to its rate of deposition. Here we describe the morphology development of a lamellae-forming poly(styrene-b-4-vinylpyridine) deposited by electrospray. Morphology was considered in the context of relative changes of the deposition and relaxation rates, with the latter significantly affected in some cases by the presence of residual solvent. We observe that the presence of residual solvent in deposited material accelerates the equilibration kinetics such that well-ordered alternating lamellar morphologies could be produced at deposition rates as high as 55 nm/min under “wet” spray conditions, whereas hexagonally packed micelles were produced when the polymer was deposited free of solvent, denoted as the “dry” spray limit. Molecular weight (MW) plays an important role in equilibration kinetics in the “dry” limit with a transition from poorly ordered to well-ordered lamellae produced by reducing MW. Film morphology was largely insensitive to temperature and flow rate over a broad range from 150 to 210 °C and from 3 to 18 μL/min respectively, although the orientation of the lamellae switched from parallel to perpendicular at elevated flow rates, potentially due to the influence of rapid solvent evaporation.
Co-reporter:Xunda Feng, Marissa E. Tousley, Matthew G. Cowan, Brian R. Wiesenauer, Siamak Nejati, Youngwoo Choo, Richard D. Noble, Menachem Elimelech, Douglas L. Gin, and Chinedum O. Osuji
ACS Nano 2014 Volume 8(Issue 12) pp:11977
Publication Date(Web):October 26, 2014
DOI:10.1021/nn505037b
There is long-standing interest in developing membranes possessing uniform pores with dimensions in the range of 1 nm and physical continuity in the macroscopic transport direction to meet the needs of challenging small molecule and ionic separations. Here we report facile, scalabe fabrication of polymer membranes with vertically (i.e., along the through-plane direction) aligned 1 nm pores by magnetic-field alignment and subsequent cross-linking of a liquid crystalline mesophase. We utilize a wedge-shaped amphiphilic species as the building block of a thermotropic columnar mesophase with 1 nm ionic nanochannels, and leverage the magnetic anisotropy of the amphiphile to control the alignment of these pores with a magnetic field. In situ X-ray scattering and subsequent optical microscopy reveal the formation of highly ordered nanostructured mesophases and cross-linked polymer films with orientational order parameters of ca. 0.95. High-resolution transmission electron microscopy (TEM) imaging provides direct visualization of long-range persistence of vertically aligned, hexagonally packed nanopores in unprecedented detail, demonstrating high-fidelity retention of structure and alignment after photo-cross-linking. Ionic conductivity measurements on the aligned membranes show a remarkable 85-fold enhancement of conductivity over nonaligned samples. These results provide a path to achieving the large area control of morphology and related enhancement of properties required for high-performance membranes and other applications.Keywords: aligned membranes; directed self-assembly; ionic conductivity; LC mesophase; magnetic alignment; nanoporous polymers;
Co-reporter:Candice I. Pelligra, Pawel W. Majewski and Chinedum O. Osuji
Nanoscale 2013 vol. 5(Issue 21) pp:10511-10517
Publication Date(Web):06 Sep 2013
DOI:10.1039/C3NR03119E
We demonstrate the use of magnetic fields for the directed assembly of ZnO nanowires in semiconducting polymer films suitable for ordered bulk heterojunction photovoltaics. Using rotational field annealing, Co-doped ZnO nanowires with negative paramagnetic anisotropy were successfully aligned out-of-plane with respect to the substrate and polymer film.
Co-reporter:Pawel W. Majewski, Manesh Gopinadhan and Chinedum O. Osuji
Soft Matter 2013 vol. 9(Issue 29) pp:7106-7116
Publication Date(Web):29 May 2013
DOI:10.1039/C3SM50320H
Magnetic fields can be highly effective in controlling the macroscale orientational order in certain uniaxial surfactant and block copolymer mesophases, leading to improvement in transport properties by reducing microstructural tortuosity. We consider ionic conductivity in model surfactant mesophases subjected to magnetic field alignment and observe significant enhancements of transport in both cylinder and lamellar forming systems, well beyond what is prescribed by microstructural models and continuum theory. Spontaneous defect generation near the order–disorder transition produces pronounced but reversible changes in conductivity on heating and cooling, implying that these pre-transitional defects are equilibrium features of the system. The enhancement and temperature dependence of conductivity closely parallel results obtained in aligned ion-containing liquid crystalline block copolymers and suggest that theoretical treatments overestimate the conductivity of the isotropic state. This is substantiated by a discussion of the underlying assumptions in the theoretical models regarding finite domain sizes and domain connectivity.
Co-reporter:Zhen Shao, Ajay Singh Negi and Chinedum O. Osuji
Soft Matter 2013 vol. 9(Issue 22) pp:5492-5500
Publication Date(Web):02 May 2013
DOI:10.1039/C3SM50209K
We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies. Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s−1. Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.
Co-reporter:Hanqiong Hu, Sofia Rangou, Myungwoong Kim, Padma Gopalan, Volkan Filiz, Apostolos Avgeropoulos, and Chinedum O. Osuji
ACS Nano 2013 Volume 7(Issue 4) pp:2960
Publication Date(Web):March 4, 2013
DOI:10.1021/nn400279a
Deposition of block copolymer thin films is most often accomplished in a serial process where material is spin coated onto a substrate and subsequently annealed, either thermally or by solvent vapor, to produce a well-ordered morphology. Here we show that under appropriate conditions, well-ordered block copolymer films may be continuously grown under substrate equilibrated conditions by slow deposition of discrete subattoliter quantities of material using electrospray. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature, block-substrate selectivity, and flow rate of the feed solution. For a PEO cylinder-forming poly(styrene-b-ethylene oxide) block copolymer, we uncover a wide temperature window from 90 to 150 °C and an ideal flow rate of 2 μL/min for ordered film deposition from dilute acetone solutions. PEO cylinders aligned with their long axes perpendicular to the film–air interface at optimal spray conditions. Using poly(styrene-b-methyl methacrylate) deposited onto neutrally selective surfaces, we show that the substrate-equilibrated process results in vertically oriented microdomains throughout the film, indicating a preservation of the initial substrate-dictated morphology during the film deposition. Electrospray offers a new and potentially exciting route for controlled, continuous growth of block copolymer thin films and manipulation of their microstructure.Keywords: block copolymer; electrospray; film growth; self-assembly; thin films
Co-reporter:Helen Tran, Manesh Gopinadhan, Pawel W. Majewski, Ryan Shade, Victoria Steffes, Chinedum O. Osuji, and Luis M. Campos
ACS Nano 2013 Volume 7(Issue 6) pp:5514
Publication Date(Web):May 20, 2013
DOI:10.1021/nn401725a
Achieving highly ordered and aligned assemblies of organic semiconductors is a persistent challenge for improving the performance of organic electronics. This is an acute problem in macromolecular systems where slow kinetics and long-range disorder prevail, thus making the fabrication of high-performance large-area semiconducting polymer films a nontrivial venture. Here, we demonstrate that the anisotropic nature of semiconducting chromophores can be effectively leveraged to yield hierarchically ordered materials that can be readily macroscopically aligned. An n-type mesogen was synthesized based on a perylene diimide (PDI) rigid core coupled to an imidazole headgroup via an alkyl spacer. Supramolecular assembly between the imidazole and acrylic acid units on a poly(styrene-b-acrylic acid) block copolymer yielded self-assembled hexagonally ordered polystyrene cylinders within a smectic A mesophase of the PDI mesogen and poly(acrylic acid). We show that magnetic fields can be used to control the alignment of the PDI species and the block copolymer superstructure concurrently in a facile manner during cooling from a high-temperature disordered state. The resulting materials are monoliths, with a single well-defined orientation of the semiconducting chromophore and block copolymer microdomains throughout the sample. This synergistic introduction of both functional properties and the means of controlling alignment by supramolecular attachment of mesogenic species to polymer backbones offer new possibilities for the modular design of functional nanostructured materials.Keywords: block copolymer self-assembly; magnetic-field alignment; n-type semiconductors; organic monoliths
Co-reporter:Shanju Zhang;Cice I. Pelligra;Gayatri Keskar;Jie Jiang;Pawel W. Majewski;André D. Taylor;Sohrab Ismail-Beigi;Lisa D. Pfefferle
Advanced Materials 2012 Volume 24( Issue 1) pp:82-87
Publication Date(Web):
DOI:10.1002/adma.201103708
Co-reporter:Meagan S. Mauter ; Menachem Elimelech
Journal of the American Chemical Society 2012 Volume 134(Issue 9) pp:3950-3953
Publication Date(Web):February 14, 2012
DOI:10.1021/ja209847u
We demonstrate the ability to stably sequester individual single-walled carbon nanotubes (SWNTs) within self-contained nanometer-scale aqueous volumes arrayed in an organic continuum. Large areal densities of 4 × 109 cm–2 are readily achieved. SWNTs are incorporated into a surfactant mesophase which forms 2.3 nm diameter water channels by lyotropic self-assembly. Near-infrared fluorescence spectroscopy demonstrates that the SWNTs exist as well-dispersed tubes that are stable over several months and through multiple cycles of heating and cooling. Absence of physical distortion of the mesophase suggests that the SWNTs are stabilized by adsorbed surfactants that do not extend considerably from the surface. Our findings have important implications for templated assembly of carbon nanotubes using soft mesophases and the development of functional nanocomposites.
Co-reporter:Manesh Gopinadhan, Pawel W. Majewski, Evan S. Beach, and Chinedum O. Osuji
ACS Macro Letters 2012 Volume 1(Issue 1) pp:184
Publication Date(Web):December 14, 2011
DOI:10.1021/mz2001059
Large-area uniform magnetic alignment of a self-assembled diblock copolymer has been achieved by the selective sequestration of rigid moieties with anisotropic diamagnetic susceptibility within one block of the system. The species is based on a biphenyl core and is confined in the acrylic acid domains of a poly(styrene-b-acrylic acid) block copolymer by hydrogen bonding between an imidazole headgroup and the acrylic acid units. Microphase separation produces hierarchically ordered systems of smectic layers within lamellae and smectic layers in the matrix surrounding hexagonally packed poly(styrene) cylinders, as a function of imidazole/acrylic acid stoichiometry. The magnetic field aligns the smectic layers as well as the block copolymer superstructure in a manner dependent on the anchoring condition of the biphenyl species at the block copolymer interface. Surprisingly, this is found to depend on the composition of the system. This approach is synergistic with recent efforts to engineer functional supramolecular block copolymer assemblies based on rigid chromophores. It offers a facile route to large area control of microstructure as required for full exploitation of functional properties in these systems.
Co-reporter:Zhen Shao, Youngjun Yang, Hyunsuk Lee, Jin Woong Kim, Chinedum O. Osuji
Journal of Colloid and Interface Science 2012 Volume 386(Issue 1) pp:135-140
Publication Date(Web):15 November 2012
DOI:10.1016/j.jcis.2012.06.085
Titania nanoparticles were modified by free-radical graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) at the particle surface, resulting in the formation of a 1–2 nm thick polymer brush. The zwitterionic nature of the polymer layer suggests that the suspension stability is a delicate function of pH, as well as volume fraction, salt concentration and the presence of charged or un-charged additives which may act as depletants or to screen charge interactions in the system. In this context, we characterized the suspension rheology as a function of volume fraction, pH, ionic strength and the concentration of surfactants in the suspension. Near-neutral pH, the brush layer is effective in stabilizing particles against aggregation with Newtonian behavior observed for volume fractions approaching 14%. Flocculation of particles and an onset of shear-thinning behavior was observed on decreasing pH from near-neutral. Conversely, suspension stability was maintained on increasing pH from near-neutral. Likewise, flocculation could be quickly induced by the addition of salt and cationic surfactant in small amounts, but the suspensions displayed greater stability to anionic and non-ionic surfactant additives. These results have important implications for the successful formulation of complex fluids employing zwitterionic colloids.Graphical abstractHighlights► Titania particles are modified using a biocompatible phosphorylcholine-based polymer brush. ► Modification provides improved biocompatibility and sensory characteristics. ► Rheology examined as function of volume fraction, pH, ionic strength, surfactant concentration. ► The suspension stability is asymmetric in its response to pH and surfactant charge.
Co-reporter:Pawel W. Majewski;Manesh Gopinadhan
Journal of Polymer Science Part B: Polymer Physics 2012 Volume 50( Issue 1) pp:2-8
Publication Date(Web):
DOI:10.1002/polb.22382
Abstract
Block copolymers (BCPs) offer an exciting range of structures and functions that are of potential utility in existing as well as emerging technologies. Although this is generally acknowledged, with few exceptions, viable strategies for establishing scalable and robust control of BCP microstructure are underdeveloped. Magnetic field alignment offers great potential in this regard. The physics bears much in common with electric field alignment, but the absence of dielectric breakdown concerns and the more flexible, space pervasive nature of magnetic fields make it possible to design processes for high-throughput fabrication of well-ordered films with appropriate materials. In this perspective, we highlight the use of magnetic fields for control of microstructure in BCPs as well as polymer nanocomposites involving anisotropic nanomaterials. A brief review of efforts to date is given. Open questions related to field-polymer interactions and future directions for magnetic alignment of these systems are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011
Co-reporter:Shanju Zhang, Candice I. Pelligra, Gayatri Keskar, Pawel W. Majewski, Fang Ren, Lisa D. Pfefferle, and Chinedum O. Osuji
ACS Nano 2011 Volume 5(Issue 10) pp:8357
Publication Date(Web):September 11, 2011
DOI:10.1021/nn203070d
Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.Keywords: dilute magnetic semiconductors; lyotropic liquid crystals; magnetic alignment; nanowires
Co-reporter:Shanju Zhang, Pawel W. Majewski, Gayatri Keskar, Lisa D. Pfefferle, and Chinedum O. Osuji
Langmuir 2011 Volume 27(Issue 18) pp:11616-11621
Publication Date(Web):July 22, 2011
DOI:10.1021/la200703u
Lyotropic nanowire dispersions are attractive precursors for semiconductor device fabrication because they permit the alignment control of active nanomaterials. The reliable production of nanowire-based mesophases, however, is very challenging in practice. We show that appropriately functionalized high-aspect-ratio nanowires of single-crystal ZnO spontaneously form nematic phases in organic and aqueous media. These systems show isotropic, biphasic, and nematic phases on increasing concentration, in reasonable agreement with Onsager’s theory for rigid rods interacting via excluded volume. Suspensions were readily processed to produce films with large-area monodomains of aligned nanowires. Imprints of the director field in quiescently dried films display a propensity for bend deformation in the organic mesophase versus splay deformation in the aqueous case, suggesting that system elasticity may be tuned via surface functionalization. These results provide critical insight for the utilization of semiconductor nanowires as novel mesogens and further enable the use of solution-based routes for fabricating optoelectronic devices.
Co-reporter:Pawel W. Majewski ; Manesh Gopinadhan ; Woo-Sik Jang ; Jodie L. Lutkenhaus
Journal of the American Chemical Society 2010 Volume 132(Issue 49) pp:17516-17522
Publication Date(Web):November 22, 2010
DOI:10.1021/ja107309p
The self-assembly of diblock copolymers provides a convenient route to the formation of mechanically robust films with precise and tunable periodic arrangements of two physically demixed but chemically linked polymeric materials. Chemoselective transport membranes may be realized from such films by selective partitioning of an active species into one of the polymer domains. Here, lithium ions were selectively sequestered within the poly(ethylene oxide) block of a liquid crystalline diblock copolymer to form polymer electrolyte membranes. Optimization of the membrane conductivity mandates alignment of self-assembled structures such that conduction occurs via direct as opposed to tortuous transport between exterior surfaces. We show here that magnetic fields can be used in a very simple and scalable manner to produce highly aligned hexagonally packed cylindrical microdomains in such membranes over macroscopic areas. We systematically explore the dependence of the ionic conductivity of the membrane on both temperature and magnetic field strength. A surprising order of magnitude increase in conductivity relative to the nonaligned case is found in films aligned at the highest magnetic field strengths, 6 T. The conductivity of field aligned samples shows a nonmonotonic dependence on temperature, with a marked decrease on heating in the proximity of the order−disorder transition of the system before increasing again at elevated temperatures. The data suggest that domain-confined transport in hexagonally packed cylindrical systems differs markedly in anisotropy by comparison with lamellar systems.
Co-reporter:Paweł W. Majewski and Chinedum O. Osuji
Langmuir 2010 Volume 26(Issue 11) pp:8737-8742
Publication Date(Web):February 25, 2010
DOI:10.1021/la100285j
We demonstrate a versatile approach to align lamellar lyotropic mesophases with the use of magnetic fields. It is based on continuous rotation of the sample on an axis perpendicular to the magnetic field direction during a single cooling ramp across the order−disorder transition of the system. The process yields materials with near-perfect, nondegenerate alignment of lamellar stacks along the axis of rotation. We use a model tetraethylene glycol dodecyl ether-water system to investigate the influence of magnetic field strength, cooling rate and the speed of sample rotation on the degree of alignment as quantitatively determined by small-angle X-ray scattering. This approach offers broad utility for the alignment of other soft mesophases relevant in several emerging applications.
Co-reporter:Meagan S. Mauter, Menachem Elimelech, and Chinedum O. Osuji
ACS Nano 2010 Volume 4(Issue 11) pp:6651
Publication Date(Web):October 18, 2010
DOI:10.1021/nn102047j
We demonstrate a novel path for the fabrication of thin-film polymer nanocomposites containing vertically aligned single-walled carbon nanotubes (SWNTs). Liquid crystal mesophases of hexagonally packed cylindrical micelles orient with their long axes parallel to an applied magnetic field and template the alignment of SWNTs sequestered in the micellar cores. The mesophase is a stable single-phase material containing monomers that can be polymerized after nanotube alignment to form the nanocomposite polymer. The space-pervasive nature of magnetic fields and the tunable physicochemical properties of multicomponent mesophases make this an attractive approach that can be leveraged for application in diverse nanocomposite systems.Keywords: alignment; liquid crystal; magnetic field; nanocomposite; single-walled carbon nanotube; SWNT; thin-film
Co-reporter:Manesh Gopinadhan, Paweł W. Majewski and Chinedum O. Osuji
Macromolecules 2010 Volume 43(Issue 7) pp:3286-3293
Publication Date(Web):March 8, 2010
DOI:10.1021/ma9026349
Large area microdomain alignment in poly(ethylene oxide-b-6-(4′-cyanobiphenyl-4-yloxy) hexyl methacrylate) block copolymers was successfully accomplished by the application of a 6 T magnetic field while cooling from elevated temperatures in the melt state. Small-angle X-ray scattering demonstrated that lamellar and cylindrical PEO microdomains aligned with their interfaces along the applied field, whereas the smectic layers of the liquid crystalline mesophase are perpendicular to the field. This is in agreement with the positive diamagnetic anisotropy of the cyano-biphenyl mesogen and a homogeneous anchoring condition at the intermaterial dividing surface (IMDS) between the two blocks. The alignment of the system is driven by the diamagnetic anisotropy of the smectic mesophase and not by the crystallization of PEO at lower temperatures. The addition of poly(acrylic acid) and LiClO4 salt result in the suppression of PEO crystallinity and stronger segregation between the polymer blocks leading to improved order in the material. The resulting films are well aligned over millimeter length scales of area and thickness. We use a novel continuous rotational annealing approach to break the degeneracy of the lamellar alignment, permitting facile directed assembly of the system during a single cooling step. Our experiments demonstrate the creation of well-aligned arrays of amorphous PEO domains over large length scales and offer a route to functional materials, in particular, for selective transport applications such as solid ionic electrolytes.
Co-reporter:Chinedum O. Osuji
Macromolecules 2010 Volume 43(Issue 7) pp:3132-3135
Publication Date(Web):March 5, 2010
DOI:10.1021/ma100066e
Co-reporter:Manesh Gopinadhan, Evan S. Beach, Paul T. Anastas and Chinedum O. Osuji
Macromolecules 2010 Volume 43(Issue 16) pp:6646-6654
Publication Date(Web):July 28, 2010
DOI:10.1021/ma1006667
We present a detailed study of the structure and properties of a supramolecular complex formed via hydrogen bond association between poly(acrylic acid) chains and an imidazole-terminated biphenyl mesogen. The system exhibits a rich phase behavior as a function of temperature and stoichiometry, expressed as the molar ratio S between the number of mesogens and binding sites present. Smectic mesophases are formed for all S ≥ 0.033, a surprisingly small number. The dependence of the characteristic length scale of the mesophase on stoichiometry does not follow the expected 1-D swelling law. At low stoichiometries, S ≤ 0.2, the system exhibits little or no change in structure up to temperatures as high as 200 °C, beyond which changes become temperature irreversible. In contrast, at higher S, the system features complex thermally driven transitions among tilted monolayer and bilayer arrangements and complete, reversible isotropization of the system at elevated temperatures. Over a limited range of temperatures and compositions, a supramolecular length scale emerges that is well beyond the upper limit imposed by a bilayer construct and thus cannot be accounted for within the conventional paradigm. Binding isotherms reveal that the polymer has a limited capacity for the ligand with saturation occurring for S ≥ 0.33. These results suggest that the common assumption of homogeneously distributed tightly bound ligands with layer-like phase separation from the polymer backbone do not apply in this system over all compositions. The anomalous phase display is consistent with demixing between polymer rich and polymer poor domains due to the presence of excess unassociated mesogen, which can act as a solvent for the system.
Co-reporter:Shanju Zhang, Lisa D. Pfefferle, and Chinedum O. Osuji
Macromolecules 2010 Volume 43(Issue 18) pp:7549-7555
Publication Date(Web):August 17, 2010
DOI:10.1021/ma101328p
We report on a series of experiments on hydrogels formed by supramolecular hairy-rod conjugated polymers and their lyotropic liquid crystalline behavior. The system is an ionic complex between negatively charged poly[2-(3-thienyl)ethyloxy-4-butylsulfonate)] backbones and cationic cetrimonium side chains. These hairy-rod supramolecules form stable isotropic solutions under dilute conditions with enhanced photoluminescence relative to the neat polymer itself. The system displays an isotropic-liquid crystalline transition on increasing concentration, resulting in the formation of a hexagonally ordered lyotropic mesophase in which the polymer chains are packed into hexagonally ordered rod-like assemblies. The mesophase is thermosensitive and can be isotropized by moderate heating. Although theoretically predicted, the observation of such lyotropic mesophases in conjugated polymers has not been reported to date. The results here are rationalized in terms of the uncharacteristically long side chains and inherent polydispersity of the system which appear necessary for stabilization of this mesophase. These results may provide new routes for the fabrication of well ordered conjugated polymer films from such solution ordered precursors for high performance electro-optic devices.
Co-reporter:Paweł W. Majewski and Chinedum O. Osuji
Soft Matter 2009 vol. 5(Issue 18) pp:3417-3421
Publication Date(Web):12 Aug 2009
DOI:10.1039/B910705C
We present a method, which we call “rotational annealing”, for the production of uniquely defined (non-degenerate) magnetic alignments of self-assembled mesophases and demonstrate its effectiveness for realizing near single-crystal quality materials over relatively short timescales.
Co-reporter:Ajay Singh Negi
Rheologica Acta 2009 Volume 48( Issue 8) pp:871-881
Publication Date(Web):2009 October
DOI:10.1007/s00397-008-0341-9
We investigate the rheology of dilute dispersions of fumed colloidal particles with attractive interactions in hydrocarbon liquids. Surprisingly, these systems display shear thickening due to the breakdown of densified flocs and a concomitant increase in the effective volume fraction of the fractal particles in the fluid. We show that this shear thickening is controlled by a critical stress and accompanied by a positive increase in the first normal stress difference, N1, at the shear thickening transition. This is in contrast to the well-known hydrocluster mechanism of shear thickening in concentrated hard-sphere and repulsive systems. Gel elasticity depends strongly on the stress applied to suspensions in preshear, scaling roughly as \(G'\sim\sigma_{\text{preshear}}^{2}\). We propose a simple model to account for these results in terms of the cluster number density determined by the preshear stress. At low shear rates, vorticity-aligned aggregates are present at \(\dot\gamma\approx 10^0 {\rm{s}}^{-1}\) . In this regime, the system displays a small but noticeable increase in viscosity on increasing shear rate. We investigate the effect of tool roughness and find that wall slip is not responsible for the observed phenomena. Instead, the increase in the apparent viscosity results from increased flow resistance due to the presence of gap-spanning log-like flocs in rolling flow.
.jpg)
