Prof. Dr. Roland Bennewitz

Inks of gold nanoparticles with stabilizing and conducting polymer shells are promising materials for printed electronics. Local measurements of their electrical properties at the single-particle scale are required to understand the relationship between the particle network and electrical functionality. Herein, we report on conductive atomic force microscopy (cAFM) on films produced from hybrid Au nanoparticles that carry a covalently bound shell of the conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and are distributed in a non-conductive matrix of polyvinyl alcohol (PVA). Current maps reveal the clustering of particles into electrically well-connected local networks and allow us to quantify the contact resistance between particles or clusters of particles. We find that the contact resistance between particles inside clusters is lower than those between clusters, indicating a hierarchical layer structure. By comparing inkjet-printed thicker bulk films and drop-cast films of single- or few-layer thickness, the experimental results offer valuable insights into the relationship between the structure of nanoparticle networks and the electrical conductance in these hybrid systems.

Individuals with more elastic, more hydrated or smaller fingers usually show better performance in several passive touch tasks. In active touch, people use different exploratory procedures when evaluating object properties, and tune their exploratory parameters. For example, they indent stimuli to assess softness and optimize their peak forces to get relevant information. In this study, we aim to understand whether finger pad size, elasticity and hydration affect individuals' force-tuning and discrimination performance in active softness perception. Participants performed two softness tasks in two different sessions. In one session, hyaluronic acid was applied to their finger pads to soften it, in the other they received no treatment. We assessed individual elasticity and hydration values with cutometer and corneometer in each session, and measured finger pad size in three dimension by caliper. In each task, two pairs of stimuli were presented to the participants (Young's Modulus: 41.5 vs. 45.0; 28.7 vs. 31.3 kPa) who chose the softer stimulus. In the restricted task, they could apply force only up to 2 Newton, whereas there was no force limit in the unconstrained task. We found that participants with smaller finger pad size exerted less force in the restricted task and participants with more hydrated and elastic fingers exerted less force in the unconstrained task. The force-tuning disappeared in the unconstrained task when treatment was applied. These results indicate that people employ strategies according to their finger parameters and to the availability of cues whereas adaptation to treatment is likely to need longer practice.

Friction was studied for the human finger pad during the spreading of viscous liquid samples in circular motion on a solid substrate. The samples included both Newtonian and shear-thinning liquids with a range of viscosity between 0.83 mPa s and 150 Pa s. During active touch, participants applied varying normal forces and sliding speeds depending on the sample and individual behavior. Friction coefficients vary greatly between participants, but fall on one Stribeck curve when shear-thinning effects were accounted for full-film lubrication. A comparison with the measured height variations during spreading demonstrates that the logarithm of the Hersey number is an instantaneous indicator of the film thickness in the full-film lubrication regime. Comparison of the measured friction coefficients with reported values of the perceived slipperiness for the same samples shows a close correspondence along the Stribeck curve.

Carbon allotropes are crucial to advanced interfaces to control friction and wear because of their unique range of mechanical properties: from diamond's hardness to graphite's lubricity. Friction force microscopy (FFM) is reported for diamond tips sliding on SiC(0001)-supported epitaxial graphene. A sharp friction increase is observed at a threshold normal force, linked to an intermittent graphene rehybridization. Comparing the FFM response of a diamond tip to that of a previously studied silicon tip with a comparable radius reveals a similar abrupt friction increase, though at roughly half the threshold force. Atomistic simulations of SiC(0001)-supported graphene sliding against hydroxylated amorphous carbon (a-C) and silicon oxide show low shear stress at low pressures for both systems. The shear stress increases at higher pressures due to bond formation between graphene and the counterbody. For a-C, the transition threshold shifts to higher pressures, consistent with FFM results. In simulations with high normal pressures, epitaxial graphene undergoes a structural transformation into single-layer diamond, contributing to the abrupt increase in friction. The graphene structure recovers after lifting the a-C counterbody, demonstrating structural robustness under tribological stress. These findings provide insights into the stability of low-friction interfaces between epitaxial graphene and key materials for current micro-electro-mechanical systems (MEMS)

Friction between fingertip and surface is a key contribution to tactile perception during active exploration of materials. We explore the role of skin factors such as stratum corneum thickness and hydration, deformability, elasticity, or density of sweat glands and of Meissner corpuscles in friction and tactile perception. The skin parameters were determined non-invasively for the glabrous skin at the index finger pad of 60 participants. Sets of randomly rough plastic surfaces and of micro-structured fibrillar rubber surfaces were explored as model materials with well-defined parameterized textures. Friction varies greatly between participants, and this variation can be explained to 70% by skin factors for the randomly rough plastic surfaces. The predictability of friction by skin factors is much lower for micro-structured rubber surfaces with bendable fibrils, where 50% of variance is explained for the stiffest fibrils but only 20% for the most bendable fibrils. The participants’ age is the key predictor for their tactile sensitivity to perceive the fibrils, where age is negatively correlated to the density of Meissner corpuscles. The results suggest that stratum corneum hydration, skin deformability, and age are important factors for friction and perception in active tactile exploration of materials.

Replica molding is a widely used technique for the fabrication of polymer microstructures. As structural dimensions decrease, anti-stick surface treatment of the mold becomes increasingly critical to ensure clean demolding and preserve structural integrity. We fabricated arrays of micropillars with 20 µm diameter and 60 µm height using medical-grade polydimethylsiloxane (PDMS), MDX4-4210, and observed a high fraction of collapsed pillars for the first molding after fluorosilanization of the mold to reduce sticking. To address this issue, we systematically investigated the surface treatment protocol for the molds, made from the PDMS Sylgard 184. We provide results from complementary measurement methods, to show that an additional vacuum step partially removes unbound fluorosilane, but does not improve pillar stability. In contrast, a method based on multiple replications, where the first replication effectively removes residual fluorosilane from the mold, significantly enhances structural stability. Mechanical testing further revealed that the presence of fluorosilane lowers the Young’s modulus of both PDMS materials, MDX4-4210 and Sylgard 184, suggesting interference with the curing process. Confocal Brillouin microscopy indicated an elongation of replicated pillars and revealed a softening close to the surfaces, as well as mechanical inhomogeneities in collapsed pillars. We discuss modifications to the molding protocol to improve the reproducibility and mechanical stability of the replicated microstructures, offering insights towards more reliable routes for the fabrication of residue-free, high-aspect ratio features with controlled surface chemistry.

There is a growing scientific interest in material unpleasantness, yet the role of distinct physical parameters in perceptual and affective haptic experiences with liquids remains to be fully understood. To address this, we investigated how perceptual qualities of liquids relate to measurable physical properties and unpleasantness during active touch. We prepared 15 custom liquid samples using everyday materials. Rheological measurements showed that samples varied between physical viscosity 1mPA s and 45 Pa s ⁠. Participants explored each sample using circular rubbing motions with their index fingers. A camera system tracked finger movements, and a force sensor revealed applied normal forces, pull-off force (PoF) and the coefficient of friction (CoF). We compared these physical properties with the perceptual dimensions from our earlier work: perceived viscosity and slipperiness. Perceived viscosity correlated strongly with both physical viscosity and PoF, but not with CoF. Conversely, perceived slipperiness was associated with CoF, but not PoF or physical viscosity, demonstrating distinct links between physics and perception of liquids. Interestingly, PoF but not CoF was significantly linked to unpleasantness, suggesting that PoF but not CoF is crucial for liquid unpleasantness. These findings advance our understanding of how distinct physical properties relate to perceptual and affective experiences of liquids.

Contact resonance atomic force microscopy (CR-AFM) has been used in many studies to characterize variations in the elastic and viscoelastic constants of materials along a heterogeneous surface. In almost all experimental work, the quantitative modulus of the surface is calculated in reference to a known reference material, rather than calculated directly from the dynamics models of the cantilever. We measured the cantilever displacement with very high sampling frequencies over the course of the experiment and captured its oscillations that result from thermal energy. Using short-term Fourier transformations, it was possible to fit the thermal resonance peak of the normal displacement to track the frequency and Q-factor of the cantilever during an experiment, using a similar process to that used to calibrate the normal bending stiffness of cantilevers. With this quantitative data, we have used the dynamic mechanics models relating the contact stiffness of the tip/cantilever pressing into a surface with the oscillation frequency of the cantilever and show that they did not accurately model the experiment. Several material combinations of tip and sample were examined; tip size and cantilever stiffness demonstrate that existing models cannot capture the physics of this problem. While concrete solutions to use analytical models to interpret CR-AFM data have not been found, a possible solution may include revisiting the analytical model to capture a potentially more complex system than the current model, improved matching the cantilever/sample stiffness to obtain a larger variation in contact stiffness with frequency, or investigating the use of higher-order modes that may achieve this improved match.

Surface-grafted polymers can reduce friction between solids in liquids by compensating the normal load with osmotic pressure, but they can also contribute to friction when fluctuating polymers entangle with the sliding counter face. We have measured forces acting on a single fluctuating double-stranded DNA polymer, which is attached to the tip of an atomic force microscope and interacts intermittently with nanometer-scale methylated pores of a self-assembled polystyrene-block-poly(4-vinylpyridine) membrane. Rare binding of the polymer into the pores is followed by a stretching of the polymer between the laterally moving tip and the surface and by a force-induced detachment. We present results for the velocity dependence of detachment forces and of attachment frequency and discuss them in terms of rare excursions of the polymer beyond its equilibrium configuration.

Fingertip friction is a key component of tactile perception. In active tactile exploration, friction forces depend on the applied normal force and on the sliding speed chosen. We have investigated whether humans perceive the speed dependence of friction for textured surfaces of materials, which show either increase or decrease of the friction coefficient with speed. Participants perceived the decrease or increase when the relative difference in friction coefficient between fast and slow sliding speed was more than 20 %. The fraction of comparison judgments which were in agreement with the measured difference in friction coefficient did not depend on variations in the applied normal force. The results indicate a perceptual constancy for fingertip friction with respect to self-generated variations of sliding speed and applied normal force.