Hans-Juergen Butt

Find an error

Name: NULL; Hans-Jürgen Butt

Organization: Max Planck Institute for Polymer Research , Germany

Department: Department of Mechanical Engineering

Title: NULL(PhD)

TOPICS

Inorganic Chemistry

Mesoporous Material

Chemicals
References

Energy Dissipation of Moving Drops on Superhydrophobic and Superoleophobic Surfaces

Co-reporter:Hans-Jürgen ButtNan Gao, Periklis Papadopoulos, Werner Steffen, Michael Kappl, Rüdiger Berger

Langmuir 2017 Volume 33(Issue 1) pp:

Publication Date(Web):November 30, 2016

DOI:10.1021/acs.langmuir.6b03792

A water drop moving on a superhydrophobic surface or an oil drop moving on a superoleophobic surface dissipates energy by pinning/depinning at nano- and microprotrusions. Here, we calculate the work required to form, extend, and rupture capillary bridges between the protrusions and the drop. The energy dissipated at one protrusion WS is derived from the observable apparent receding contact angle Θrapp and the density of protrusions n by Ws = γ(cos Θrapp + 1)/n, where γ is the surface tension of the liquid. To derive an expression for Ws that links the microscopic structure of the surface to apparent contact angles, two models are considered: A superhydrophobic array of cylindrical micropillars and a superoleophobic array of stacks of microspheres. For a radius of a protrusion R and a receding materials contact angle Θr, we calculate the energy dissipated per protrusion as Ws = πγR2[A – ln(R/κ)]f(Θr). Here, A = 0.60 for cylindrical micropillars and 2.9 for stacks of spheres. κ is the capillary length. f(Θr) is a function which depends on Θr and the specific geometry, f ranges from ≈0.25 to 0.96. Combining both equations above, we can correlate the macroscopically observed apparent receding contact angle with the microscopic structure of the surface and its material properties.

Stable Hydrophobic Metal-Oxide Photocatalysts via Grafting Polydimethylsiloxane Brush

Co-reporter:Sanghyuk Wooh;Noemí Encinas;Doris Vollmer;Hans-Jürgen Butt

Advanced Materials 2017 Volume 29(Issue 16) pp:

Publication Date(Web):2017/04/01

DOI:10.1002/adma.201604637

Polydimethylsiloxane (PDMS) can be grafted to metal-oxide photocatalysts such as titanium oxide by simple UV irradiation in solution or melt. The PDMS graft metal oxides are still photocatalytically active. They are hydrophobic, liquid repellent, self-cleaning, prevent biofouling and are long-term stable even in UV light.

A Photocatalytically Active Lubricant-Impregnated Surface

Co-reporter:Dr. Sanghyuk Wooh; Dr. Hans-Jürgen Butt

Angewandte Chemie 2017 Volume 129(Issue 18) pp:5047-5051

Publication Date(Web):2017/04/24

DOI:10.1002/ange.201611277

AbstractLubricant impregnated surfaces (LISs) exhibit sliding angles below 5°. A LIS is presented that possesses photocatalytic activity as well as improved liquid repellency. In a single-step reaction, the surface of photocatalytic mesoporous TiO2 substrate is modified by grafting polydimethylsiloxane (PDMS) brush and the residual non-bound PDMS serves as lubricant. Since the lubricant and the hydrophobic layer are chemically identical, the grafting PDMS layer is stably swollen by the lubricant PDMS, which inhibits direct contact of liquid drops to the solid substrate. Liquid drops such as water, methanol, and even low-surface-tension fluorocarbons, slide on the surface with tilt angles below 1°. The surface exhibits long-term stable photocatalytic activity while retaining its liquid repellency. This photocatalytic activity allows photocatalytic chemistry, for example, decomposition of organics, on LIS to be carried out.

Recent experimental advances for understanding bubble-particle attachment in flotation

Co-reporter:Yaowen Xing, Xiahui Gui, Lei Pan, Bat-El Pinchasik, Yijun Cao, Jiongtian Liu, Michael Kappl, Hans-Jürgen Butt

Advances in Colloid and Interface Science 2017 Volume 246(Volume 246) pp:

Publication Date(Web):1 August 2017

DOI:10.1016/j.cis.2017.05.019

•Bubble-particle attachment is fundamental for understanding flotation process.•Advances in macroscopic experimental approaches to flotation are reviewed.•Nanoscale techniques to probe surface forces and liquid film drainage are reviewed.•Synchronous detection of forces and film drainage profile is a promising trend.Bubble-particle interaction is of great theoretical and practical importance in flotation. Significant progress has been achieved over the past years and the process of bubble-particle collision is reasonably well understood. This, however, is not the case for bubble-particle attachment leading to three-phase contact line formation due to the difficulty in both theoretical analysis and experimental verification. For attachment, surface forces play a major role. They control the thinning and rupture of the liquid film between the bubble and the particle. The coupling between force, bubble deformation and film drainage is critical to understand the underlying mechanism responsible for bubble-particle attachment. In this review we first discuss the advances in macroscopic experimental methods for characterizing bubble-particle attachment such as induction timer and high speed visualization. Then we focus on advances in measuring the force and drainage of thin liquid films between an air bubble and a solid surface at a nanometer scale. Advances, limits, challenges, and future research opportunities are discussed. By combining atomic force microscopy and reflection interference contrast microscopy, the force, bubble deformation, and liquid film drainage can be measured simultaneously. The simultaneous measurement of the interaction force and the spatiotemporal evolution of the confined liquid film hold great promise to shed new light on flotation.Download high-res image (187KB)Download full-size image

Shape of a sessile drop on a flat surface covered with a liquid film

Co-reporter:Martin Tress;Stefan Karpitschka;Periklis Papadopoulos;Jacco H. Snoeijer;Doris Vollmer;Hans-Jürgen Butt

Soft Matter (2005-Present) 2017 vol. 13(Issue 20) pp:3760-3767

Publication Date(Web):2017/05/24

DOI:10.1039/C7SM00437K

Motivated by the development of lubricant-infused slippery surfaces, we study a sessile drop of a nonvolatile (ionic) liquid which is embedded in a slowly evaporating lubricant film (n-decane) on a horizontal, planar solid substrate. Using laser scanning confocal microscopy we imaged the evolution of the shape of the liquid/liquid and liquid/air interfaces, including the angles between them. Results are compared to solutions of the generalized Laplace equations describing the drop profile and the annular wetting ridge. For all film thicknesses, experimental results agree quantitatively with the calculated drop and film shapes. With the verified theory we can predict height and volume of the wetting ridge. Two regimes can be distinguished: for macroscopically thick films (excess lubrication) the meniscus size is insensitive to changes in film thickness. Once the film is thin enough that surface forces between the lubricant/air and solid/lubricant interfaces become significant the meniscus changes significantly with varying film thickness (starved lubrication). The size of the meniscus is particularly relevant because it affects sliding angles of drops on lubricant-infused surfaces.

Controlling the Structure of Supraballs by pH-Responsive Particle Assembly

Co-reporter:Takafumi Sekido, Sanghyuk WoohRegina Fuchs, Michael Kappl, Yoshinobu Nakamura, Hans-Jürgen Butt, Syuji Fujii

Langmuir 2017 Volume 33(Issue 8) pp:

Publication Date(Web):February 8, 2017

DOI:10.1021/acs.langmuir.6b04648

Supraballs of various sizes and compositions can be fabricated via drying of drops of aqueous colloidal dispersions on super-liquid-repellent surfaces with no chemical waste and energy consumption. A “supraball” is a particle composed of colloids. Many properties, such as mechanical strength and porosity, are determined by the ordering of a colloidal assembly. To tune such properties, a colloidal assembly needs to be controlled when supraballs are formed during drying. Here, we introduce a method to control a colloidal assembly of supraballs by adjusting the dispersity of the colloids. Supraballs are fabricated on superamphiphobic surfaces from colloidal aqueous dispersions of polystyrene microparticles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate]. Drying of dispersion drops at pH 3 on superamphiphobic surfaces leads to the formation of spherical supraballs with densely packed colloids. The pH 10 supraballs are more oblate and consist of more disordered colloids than the pH 3 supraballs, caused by particle aggregates with random sizes and shapes in the pH 10 dispersion. Thus, the shape, crystallinity, porosity, and mechanical properties could be controlled by pH, which allows broader uses of supraballs.

Near-Infrared-Sensitive Materials Based on Upconverting Nanoparticles

Co-reporter:Si Wu;Hans-Jürgen Butt

Advanced Materials 2016 Volume 28( Issue 6) pp:1208-1226

Publication Date(Web):

DOI:10.1002/adma.201502843

The near-infrared (NIR) region of the spectrum is called the “therapeutic window” because NIR light can penetrate deeply into tissue. Therefore, NIR-sensitive materials are attractive for biomedical applications. Recently, upconverting nanoparticles (UCNPs) were used to construct NIR-sensitive materials. UCNPs convert NIR light to UV or visible light, which can trigger photoreactions of photosensitive materials. Here, how to use UCNPs to construct NIR-sensitive materials is introduced, applications of NIR-sensitive materials with a focus on biomedical applications are highlighted, and the associated challenges are discussed.

Mechanical Properties of Highly Porous Super Liquid-Repellent Surfaces

Co-reporter:Maxime Paven;Regina Fuchs;Taro Yakabe;Doris Vollmer;Michael Kappl;Akiko N. Itakura;Hans-Jürgen Butt

Advanced Functional Materials 2016 Volume 26( Issue 27) pp:4914-4922

Publication Date(Web):

DOI:10.1002/adfm.201600627

Surfaces with self-cleaning properties are desirable for many applications. Conceptually, super liquid-repellent surfaces are required to be highly porous on the nano- or micrometer scale, which inherently makes them mechanically weak. Optimizing the balance of mechanical strength and liquid repellency is a core aspect toward applications. However, quantitative mechanical testing of porous, super liquid-repellent surfaces is challenging due to their high surface roughness at different length scales and low stress tolerance. For this reason, mechanical testing is often performed qualitatively. Here, the mechanical responses of soot-templated super liquid-repellent surfaces are studied qualitatively by pencil and finger scratching and quantitatively by atomic force microscopy, colloidal probe force measurements, and nanoindentation. In particular, colloidal probe force measurements cover the relevant force and length scales. The effective elastic modulus, the plastic work W_plastic and the effective adhesive work W_adhesive are quantified. By combining quantitative information from force measurements with measurements of surface wetting properties, it is shown that mechanical strength can be balanced against low wettability by tuning the reaction parameters.

Synthesis of Mesoporous Supraparticles on Superamphiphobic Surfaces

Co-reporter:Sanghyuk Wooh;Hannah Huesmann;Muhammad Nawaz Tahir;Maxime Paven;Kristina Wichmann;Doris Vollmer;Wolfgang Tremel;Periklis Papadopoulos;Hans-Jürgen Butt

Advanced Materials 2015 Volume 27( Issue 45) pp:7338-7343

Publication Date(Web):

DOI:10.1002/adma.201503929

Super liquid-repellent layers: The smaller the better

Co-reporter:Hans-Jürgen Butt, Doris Vollmer, Periklis Papadopoulos

Advances in Colloid and Interface Science 2015 Volume 222() pp:104-109

Publication Date(Web):August 2015

DOI:10.1016/j.cis.2014.06.002

Highlights

•: Superhydrophobic and superoleophobic structures should be as small as possible.
•: Scaling down superamphiphobic surfaces leads to a high impalement pressure and a high contact angle.
•: The apparent receding contact angle is related to the capillary force of individual pillars.

Direct observation of drops on slippery lubricant-infused surfaces

Co-reporter:Frank Schellenberger, Jing Xie, Noemí Encinas, Alexandre Hardy, Markus Klapper, Periklis Papadopoulos, Hans-Jürgen Butt and Doris Vollmer

Soft Matter 2015 vol. 11(Issue 38) pp:7617-7626

Publication Date(Web):13 Aug 2015

DOI:10.1039/C5SM01809A

For a liquid droplet to slide down a solid planar surface, the surface usually has to be tilted above a critical angle of approximately 10°. By contrast, droplets of nearly any liquid “slip” on lubricant-infused textured surfaces – so termed slippery surfaces – when tilted by only a few degrees. The mechanism of how the lubricant alters the static and dynamic properties of the drop remains elusive because the drop–lubricant interface is hidden. Here, we image the shape of drops on lubricant-infused surfaces by laser scanning confocal microscopy. The contact angle of the drop–lubricant interface with the substrate exceeds 140°, although macroscopic contour images suggest angles as low as 60°. Confocal microscopy of moving drops reveals fundamentally different processes at the front and rear. Drops recede via discrete depinning events from surface protrusions at a defined receding contact angle, whereas the advancing contact angle is 180°. Drops slide easily, as the apparent contact angles with the substrate are high and the drop–lubricant interfacial tension is typically lower than the drop–air interfacial tension. Slippery surfaces resemble superhydrophobic surfaces with two main differences: drops on a slippery surface are surrounded by a wetting ridge of adjustable height and the air underneath the drop in the case of a superhydrophobic surface is replaced by lubricant in the case of a slippery surface.

Interaction between Air Bubbles and Superhydrophobic Surfaces in Aqueous Solutions

Co-reporter:Chen Shi, Xin Cui, Xurui Zhang, Plamen Tchoukov, Qingxia Liu, Noemi Encinas, Maxime Paven, Florian Geyer, Doris Vollmer, Zhenghe Xu, Hans-Jürgen Butt, and Hongbo Zeng

Langmuir 2015 Volume 31(Issue 26) pp:7317-7327

Publication Date(Web):June 12, 2015

DOI:10.1021/acs.langmuir.5b01157

Superhydrophobic surfaces are usually characterized by a high apparent contact angle of water drops in air. Here we analyze the inverse situation: Rather than focusing on water repellency in air, we measure the attractive interaction of air bubbles and superhydrophobic surfaces in water. Forces were measured between microbubbles with radii R of 40–90 μm attached to an atomic force microscope cantilever and submerged superhydrophobic surfaces. In addition, forces between macroscopic bubbles (R = 1.2 mm) at the end of capillaries and superhydrophobic surfaces were measured. As superhydrophobic surfaces we applied soot-templated surfaces, nanofilament surfaces, micropillar arrays with flat top faces, and decorated micropillars. Depending on the specific structure of the superhydrophobic surfaces and the presence and amount of entrapped air, different interactions were observed. Soot-templated surfaces in the Cassie state showed superaerophilic behavior: Once the electrostatic double-layer force and a hydrodynamic repulsion were overcome, bubbles jumped onto the surface and fully merged with the entrapped air. On nanofilaments and micropillar arrays we observed in addition the formation of sessile bubbles with finite contact angles below 90° or the attachment of bubbles, which retained their spherical shape.

Foreword

Co-reporter:Victor M. Starov, Hans-Jürgen Butt

Current Opinion in Colloid & Interface Science 2014 Volume 19(Issue 4) pp:253-254

Publication Date(Web):August 2014

DOI:10.1016/j.cocis.2014.11.004

Floating on Oil

Co-reporter:Jihua Zhang, Xu Deng, Hans-Jürgen Butt, and Doris Vollmer

Langmuir 2014 Volume 30(Issue 35) pp:10637-10642

Publication Date(Web):2017-2-22

DOI:10.1021/la502426x

We demonstrate that disk-shaped steel meshes coated with a superamphiphobic layer are able to float on water and on organic liquids. A coated disk-shaped steel mesh of 1 cm radius has a loading capacity of 17 mN in water and still remarkable 9 mN in n-hexadecane. Experimentally measured supporting forces and loading capacities agree well with theoretical predictions. Inspired by the giant water lily, pan-shaped “oil lilies” with even higher loading capacity and artificial oil striders carrying more than 10 times their own weight are designed. Even after the artificial devices are fully immersed into different liquids, they show self-draining properties due to capillary forces.

Transparent and airtight silica nano- and microchannels with uniform tubular cross-section

Co-reporter:Lena Mammen, Periklis Papadopoulos, Kathrin Friedemann, Stefanie Wanka, Daniel Crespy, Doris Vollmer and Hans-Jürgen Butt

Soft Matter 2013 vol. 9(Issue 41) pp:9824-9832

Publication Date(Web):15 Aug 2013

DOI:10.1039/C3SM51082D

Experimental results on a template-assisted technique to fabricate uniform nano- and microchannels are presented. Different template materials, polystyrene and poly(vinyl alcohol) electrospun and spider silk fibres were coated with silica. After calcination mechanically stable and transparent channels with uniform tubular diameter were obtained. The diameters ranged between 150 nm and 4 μm, depending on the size of the fibre template. By coating crossed fibres we prepared connected channel junctions that were solvent- and airtight. The channels and junctions remained tight even when applying a pressure above 3 bar as verified by laser scanning confocal microscopy. The flow of liquids in the channels was monitored and described by the Lucas–Washburn equation. We varied the viscosities and surface tensions of the liquids and measured the filling velocities over a distance of several millimeters as well as very close to the channel entrance. The late stage of capillary filling can be described well by the Lucas–Washburn equation. The early stage was slower than predicted. We attribute the delayed filling to the velocity dependence of the contact angle and modeled the filling behaviour by dynamic wetting theories.

Design principles for superamphiphobic surfaces

Co-reporter:Hans-Jürgen Butt, Ciro Semprebon, Periklis Papadopoulos, Doris Vollmer, Martin Brinkmann and Matteo Ciccotti

Soft Matter 2013 vol. 9(Issue 2) pp:418-428

Publication Date(Web):23 Oct 2012

DOI:10.1039/C2SM27016A

To predict the properties of superamphiphobic layers we analyzed the wetting of a square and a hexagonal array of vertical pillars composed of spheres (radius R) partially sintered together. Apparent contact angles above 150° are obtained by pinning of a non-polar liquid surface at the underside of the top sphere resulting in a Fakir or Cassie state. Analytical equations are derived for the impalement pressure in the limiting case A0 ≫ R2, where A0 is the area of the regular unit cell containing a single pillar. The case of close pillars is investigated numerically. By balancing forces at the rim of a drop, we calculate the apparent receding contact angle. To describe drag reduction of a flowing liquid we calculate the apparent slip length. When considering pressure-induced flow through cylindrical capillaries of radius rc, significant drag reduction occurs only for thin capillaries. The mechanical stability with respect to normal forces and shear is analyzed. Nanoscopic silica glass pillars would be able to sustain the normal and shear stresses caused by capillary and drag forces. For a high impalement pressure and good mechanical stability A0 should be small and R (respectively the neck diameter) should be large, whereas a large A0 and a small R imply low contact angle hysteresis and high slip length.

Liquid Drops Impacting Superamphiphobic Coatings

Co-reporter:Xu Deng, Frank Schellenberger, Periklis Papadopoulos, Doris Vollmer, and Hans-Jürgen Butt

Langmuir 2013 Volume 29(Issue 25) pp:7847-7856

Publication Date(Web):May 22, 2013

DOI:10.1021/la401120j

The dynamics of liquid drops impacting superamphiphobic coatings is studied by high-speed video microscopy. Superamphiphobic coatings repel water and oils. The coating consists of a fractal-like hydrophobized silica network. Mixtures of ethanol–water and glycerin–water are chosen to investigate the influence of interfacial tension and viscosity on spreading and retraction dynamics. Drop spreading is dominated by inertia. At low impact velocity, the drops completely rebound. However, the contact time increases with impact velocity, whereas the restitution coefficient decreases. We suggest that the drop temporarily impales the superamphiphobic coating, although the drop completely rebounds. From an estimate of the pressure, it can be concluded that impalement is dominated by depinning rather than sagging. With increasing velocity, the drops partially pin, and an increasing amount of liquid remains on the coating. A time-resolved study of the retraction dynamics reveals two well-separated phases: a fast inertia-dominated phase followed by a slow decrease of the contact diameter of the drop. The crossover occurs when the diameter of the retracting drop matches the diameter of the drop before impact. We suggest that the depth of impalement increases with impact velocity, where impalement is confined to the initial impact zone of the drop. If the drop partially pins on the coating, the depth of impalement exceeds a depth, preventing the whole drop from being removed during the retraction phase.

Stimuli-Responsive Y-Shaped Polymer Brushes Based on Junction-Point-Reactive Block Copolymers

Co-reporter:Christoph Tonhauser;Ali A. Golriz;Christian Moers;Rebecca Klein;Hans-Jürgen Butt;Holger Frey

Advanced Materials 2012 Volume 24( Issue 41) pp:5559-5563

Publication Date(Web):

DOI:10.1002/adma.201202105

Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids

Co-reporter:Pranesh Muralidhar;Elmar Bonaccurso;Günter K. Auernhammer

Colloid and Polymer Science 2011 Volume 289( Issue 14) pp:

Publication Date(Web):2011 September

DOI:10.1007/s00396-011-2475-z

The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene) have been studied with a high-speed camera. For drops of 1–4 μL volume, the increase in contact radius r can be described by a power law \( r \propto {t^{\alpha }} \), with the spreading exponent α = 0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact angle of Θ0 = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ0 = 0) and water (α = 0.35, Θ0 = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.

Capillary forces between soft, elastic spheres

Co-reporter:Hans-Jürgen Butt, W. Jon P. Barnes, Aranzazu del Campo, Michael Kappl and Friedhelm Schönfeld

Soft Matter 2010 vol. 6(Issue 23) pp:5930-5936

Publication Date(Web):20 Sep 2010

DOI:10.1039/C0SM00455C

The adhesive capillary force between two elastic spheres or a sphere and a plane is usually described by Fadh = 4πR*, where γ is the surface tension of the liquid and R* is the effective radius of the sphere(s). With approximate analytical calculations we show that for soft materials and radii larger than 5r3E2/γ2 (r: radius of curvature of the liquid and E: Young's modulus) the capillary force increases more steeply and scales with R*2. This result is confirmed by finite element simulations. It may be relevant for an understanding of bioadhesion of insects and tree frogs and for an understanding of film formation from dispersions of soft particles.

Studying mechanical microcontacts of fine particles with the quartz crystal microbalance

Co-reporter:Ewa Vittorias, Michael Kappl, Hans-Jürgen Butt, Diethelm Johannsmann

Powder Technology 2010 Volume 203(Issue 3) pp:489-502

Publication Date(Web):25 November 2010

DOI:10.1016/j.powtec.2010.06.011

To study micromechanical adhesion, glass particles were deposited on a quartz crystal microbalance (QCM). Beforehand, a 160 nm-thick film of polystyrene (PS) had been spin-coated on the gold surface of the QCM. Shifts in the resonance frequency were monitored versus the oscillation amplitude. The aim was to analyse how QCM experiments reflect the state of adhesion. During oscillation, the motion of the particles and the induced frequency shift of the QCM are governed by a balance between inertial and contact forces. In order to vary the relative strength of the two, the diameter of the particles was varied between 5 and 20 μm. The adherence of the particles could be increased by annealing the PS film at 150 °C. Annealing led to the formation of a PS meniscus. For a semi-quantitative interpretation we have to take into account that the particles show a distribution of coupling constants.The vibration of the QCM changes the micromechanical contact between QCM surface and particles. There is an instantaneous and a long-term effect. Instantaneously, the oscillation induces partial slip. Under an oscillating load, part of the contact ruptures, which decreases the effective stiffness of the contact. In addition, there are long-term memory effects. The vibration of the QCM can lead to a consolidation and an increased coupling. However, it can also break the contact and even lead to detachment. Particles deform the PS surface and induce damage due to inertial forces.To study micromechanical adhesion, particles of different size (R = 2.5–10 μm) were deposited on a quartz crystal microbalance (QCM). Adhesion was varied by coating the surface of the QCM. From the shift in resonance frequency, information on the interaction between particles and the surface of the QCM is provided.

Effect of Humidity on Nanoscale Adhesion on Self-Assembled Thiol Monolayers Studied by Dynamic Force Spectroscopy

Co-reporter:Hubert Gojzewski, Michael Kappl, Arkadiusz Ptak and Hans-Jürgen Butt

Langmuir 2010 Volume 26(Issue 3) pp:1837-1847

Publication Date(Web):October 1, 2009

DOI:10.1021/la902559n

The adhesion force between silicon nitride tips of an atomic force microscope and different self-assembled thiol monolayers (SAMs) was measured at different loading rates and humidity. SAMs were formed from HS(CH2)nCH3 with n = 6, 8, 9, 10, 15 and HS(CH2)nOH with n = 6, 9, 11, 16. With a special setup, the loading rate could be increased to 107 nN s−1. For the interaction with two-dimensional crystalline CH3-terminated SAMs (n ≥ 8), two regimes can be distinguished. At loading rates below 104−105 nN s−1, the adhesion force increased proportional to the logarithm of the loading rate. Adhesion is most likely dominated by van der Waals attraction. At higher loading rates, the adhesion forces increased steeper with the logarithm of the loading rate. The specific process limiting separation is not yet identified. On OH-terminated SAMs, the adhesion force was ∼6 times higher than on the CH3-terminated SAMs, even at low humidity. This can partially, but not fully, be explained by hydrogen bridges forming between the hydroxyl groups of the monolayer and silanol groups of the tip. For relative humidity above 10%, the capillary force further increased the adhesion force, which reached a maximum at values of relative humidity between 40% and 80%. Adhesion force versus loading rate (Fad versus rF) curves increased roughly linearly over the whole range of loading rates. The slope depended on the humidity, and it is correlated with the absolute strength of the capillary force.

Dynamic Wetting of Polyisoprene Melts: Influence of the End Group

Co-reporter:Chuanjun Liu, Elmar Bonaccurso, Mordechai Sokuler, Günter K. Auernhammer and Hans-Jürgen Butt

Langmuir 2010 Volume 26(Issue 4) pp:2544-2549

Publication Date(Web):November 3, 2009

DOI:10.1021/la9028194

The spontaneous spreading of drops of polyisoprene melt terminated with methyl (PI−CH3), hydroxyl (PI−OH), and carboxyl groups (PI−COOH) on hydrophilic silicon surfaces has been studied experimentally. Despite the fact that all three polymers have a similar surface tension (0.032 N/m) at the polymer−air interface, the equilibrium contact angles were 29°, <5°, and 20°, respectively. Spreading of PI−OH and PI−COOH is slowed down as compared to PI−CH3, most probably due to the strong interfacial binding of the hydroxyl or carboxyl end group to the surface. We interpret and discuss the dynamic wetting experiments using the hydrodynamic and molecular kinetic theory of wetting.

Interaction of a Microsphere with a Solid-Supported Liquid Film

Co-reporter:Javed Ally, Ewa Vittorias, A. Amirfazli, Michael Kappl, Elmar Bonaccurso, Cathy E. McNamee and Hans-Jürgen Butt

Langmuir 2010 Volume 26(Issue 14) pp:11797-11803

Publication Date(Web):June 22, 2010

DOI:10.1021/la1010924

The interaction between particles with thin liquid films on solid surfaces was studied by sintering polystyrene microspheres of 4 to 5 μm diameter to the end of atomic force microscope cantilevers. Films of three silicone oils (viscosity 4.6, 9.2, and 9700 mPa s) and water of thickness 0.2−1.8 μm were formed on glass. The interaction between a particle and the film was measured at different particle approach/retraction velocities. The interaction is dominated by capillary and hydrodynamic forces. It depends on the surface tension and the viscosity of the liquid. The film thickness can be determined from the force curves. In addition, the meniscus formation of a film wetting a particle was demonstrated experimentally by solidifying a liquid polystyrene film as it wetted glass particles.

Normal capillary forces

Co-reporter:Hans-Jürgen Butt, Michael Kappl

Advances in Colloid and Interface Science 2009 Volume 146(1–2) pp:48-60

Publication Date(Web):28 February 2009

DOI:10.1016/j.cis.2008.10.002

Abstract

A liquid meniscus between two lyophilic solid surfaces causes an attractive force, the capillary force. The meniscus can form by capillary condensation or by accumulation of adsorbed liquid. Under ambient conditions and between hydrophilic surfaces, capillary forces usually dominate over other surface forces. They are relevant in many processes occurring in nature and technical applications, for example the flow of granular materials and friction between surfaces. Here we review normal capillary forces, focusing on a quantitative description with continuum theory. After introducing the capillary force between spherical surfaces, we extend the discussion to other regular and irregular surfaces. The influence of surface roughness is considered. In addition to capillary forces at equilibrium, we also describe the process of meniscus formation. Assumptions, limits, and perspectives for future work are discussed.

Influence of Binding-Site Density in Wet Bioadhesion

Co-reporter:Jijun Wang;Muhammad Nawaz Tahir;Michael Kappl;Wolfgang Tremel;Nadine Metz;Matthias Barz;Patrick Theato;Hans-Jürgen Butt

Advanced Materials 2008 Volume 20( Issue 20) pp:3872-3876

Publication Date(Web):

DOI:10.1002/adma.200801140

Evaporation Structures of Solvent Drops Evaporating from Polymer Surfaces: Influence of Molar Mass

Co-reporter:Guangfen Li;Dmytro S. Golovko;Karlheinz Graf;Elmar Bonaccurso;Andreas Best;Hans-Jürgen Butt

Macromolecular Chemistry and Physics 2007 Volume 208(Issue 19‐20) pp:2134-2144

Publication Date(Web):10 OCT 2007

DOI:10.1002/macp.200700122

When a solvent drop evaporates from a polymer surface, a characteristic structure remains. To analyze the structure formation during evaporation we deposited microdrops of ethyl acetate on planar PEMA and toluene on PS. The shape of the evaporation structures depends on the molar mass of the polymer, the number of droplets deposited, and the specific polymer/solvent combination. Crater-like structures with a flat bottom were observed for PEMA. With PS, dot-like protrusions were observed for low and intermediate molar masses; for ≥ 210 kDa, crater-like rims with a depression in the center were formed. These structures are interpreted based on four different processes occurring during evaporation.

Control of surface properties of self-assembled monolayers by tuning the degree of molecular asymmetry

Co-reporter:Susana Moreno Flores, Andrey Shaporenko, Chandrasekhar Vavilala, Hans-Jürgen Butt, Michael Schmittel, Michael Zharnikov, Rüdiger Berger

Surface Science 2006 Volume 600(Issue 14) pp:2847-2856

Publication Date(Web):15 July 2006

DOI:10.1016/j.susc.2006.04.047

We have studied self-assembled monolayers (SAMs) of asymmetric dialkyldisulfide derivatives of the form CH3–(CH2)11+m–S–S–(CH2)11–OH with m = −4, −3, 0, +2 and +4 on gold. Sub-nanoscale changes in the length of the CH3-terminated alkylchain have been used to selectively protrude one particular end group in the resulting film. The alteration of the chain length in only two methylene units already results in changes of surface properties, which have been detected with local (chemical force microscopy) and macroscopic (contact angle) techniques. In particular, advancing contact angles can be adjusted between 40° and 80°. The adhesion between a hydrophobic tip and these SAMs in water is determined by the chemical nature of the protruding end group. Chemical force microscopy, X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy have shown that these SAMs are composed of mixed, well-packed CH3– and OH–alkylthiolate branches. The surface composition ratio is close to 1:1 for all investigated SAMs.

Towards Powering Nanometer-Scale Devices with Molecular Motors: Single Molecule Engines

Co-reporter:Hans-Jürgen Butt

Macromolecular Chemistry and Physics 2006 Volume 207(Issue 6) pp:573-575

Publication Date(Web):6 MAR 2006

DOI:10.1002/macp.200600059

Summary: Future nanodevices such as levers, valves, pumps, and other functional parts will require molecular motors to generate mechanical work. Electrochemically switchable single polymer chains are possible candidates for such motors. The proof of principle that such a single macromolecular motor can work has been achieved using poly(ferrocenyldimethylsilane) (PFS). PFS can reversibly be oxidized and reduced by an external potential. This leads to changes in its mechanical properties, which can be used to drive a cyclic molecular engine. Such a system is compatible with a high density array of individually addressable molecular motors, e.g. by interfacing with electrodes smaller than the diffraction limit of light.

Forces between a stiff and a soft surface

Co-reporter:Hans-Jürgen Butt, Jonathan T. Pham, Michael Kappl

Current Opinion in Colloid & Interface Science (February 2017) Volume 27() pp:

Publication Date(Web):February 2017

DOI:10.1016/j.cocis.2016.09.007

The contact between a sphere and a planar half space, one being rigid and the other elastic (or between two elastic spheres), can be described by the JKR theory of Johnson, Kendall and Roberts (Proc. R. Soc. Lond. A 1971, 324, 301). One assumption of JKR theory is that the characteristic length scale L ≈ w/E is much smaller than the radius R of the sphere; where w is the work of adhesion and E is the Young's modulus of the soft, elastic body. Relative deformations for a mechanical contact increase with increasing L and decreasing particle size R. Experiments show that up to at least L/R = 0.2, JKR theory predicts the correct dependencies between the contact radius, the indentation and the load. However, when R ≫ L is no longer satisfied, the change in total free surface area due to deformation needs to be considered. Then, elastocapillary effects start playing a significant role. In addition to discussing theory and experiments of pure solid contacts, the effect of elastic deformation on capillary and hydrodynamic forces is discussed. Finally, we consider the interaction of hollow capsules as one example of a deformable body that is still formed from a stiff material.