Interaktive Oberflächen

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.

Our perception does not consist of isolated events but unfold and adapt over time, forming a continuous stream of experience. How we perceive an object is influenced not only by its intrinsic characteristics but also by our recent interactions, leading to sequential effects (i.e., serial dependence). In this study, we investigated whether such effects occur in haptic material perception, specifically in pleasantness and roughness. We also examined whether serial dependence is grounded in the physical object properties or the task at hand (i.e., perceptual attribute). To this end, we used seven sandpaper samples ranging in roughness (18.3 to 425 microns). In two separate sessions, individuals explored the samples with their index finger and rated how pleasant or rough each object feels. Our results revealed a significant attractive serial dependence in roughness judgments and a weaker repulsive serial dependence in pleasantness judgments. This shows distinct patterns of serial dependence for roughness and pleasantness emerging for the same physical stimuli. These divergent patterns for the same stimuli suggest that serial dependence in haptics is attribute dependent, reflecting higher order mechanism rather than being purely driven by stimulus characteristics. Recognizing the role of serial dependence in haptics opens new avenues for haptic interfaces and engineering user experiences.

Abstract Skin physiology varies greatly between anatomical locations, with impact on tactile
sensitivity. Research on the variations in morphology and physiology between different anatomical
locations of glabrous skin is limited so far. This study investigates the morphology and physiology
of glabrous skin on the human hand (N = 60), including the index finger pads of dominant and
non-dominant hand, the small finger pads, the thenar palm and the volar arm as hairy skin reference.
We employed laser scanning microscopy, optical coherence tomography, skin corneometry, skin
cutometry and two-point discrimination measurements as non-invasive in situ methods. We found
significant morphological and physiological differences between fingers, palm and volar arm. The
finger pads had a thicker stratum corneum (SC), especially the index finger of the dominant hand.
They also exhibited higher skin hydration and a higher density of Meissner corpuscles and of
sweat glands compared to the palm. All parameters exhibited strong interindividual variability.
A correlation between anatomical positions for individuals was found for some but not all
parameters. The skin’s mechanical response differed significantly between the anatomical locations,
in correlation with skin hydration for all glabrous skin positions. The effect of ageing on skin elasticity
was strongest on the hairy skin of the volar arm. In glabrous skin ageing affected the density of
Meissner corpuscles and thus spatial acuity. Glabrous skin exhibits a high specialization depending
on the anatomical position. Physiological and morphological parameters play a crucial role in its
mechanical properties and tactile acuity.

The perceived time can shrink or expand for emotional stimuli. Converging evidence suggests that emotional
time distortions are rooted in the emotional states of the timing agents because emotional stimuli can influence
the timing of simultaneous neutral events. As emotional states are transitory, we investigated if time modulating
emotional states also influence timing of subsequent neutral events. In each trial, we induced different valence
and arousal levels by using affective vibrotactile patterns before participants judged the duration of neutral
auditory tones. Compared to neutral patterns, affective patterns modulated participants’ time perception of the
subsequent tones. We observed an interaction between arousal and valence: Pleasant-Low arousal patterns
expanded the timing of subsequent neutral events more than Unpleasant-Low arousal patterns while Pleasant and
Unpleasant-High arousal led to a similar temporal expansion. Our results indicate time modulating effects of
emotional stimuli are due to changed emotional states and influence time perception likely until the underlying
state decays.

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.

Previous studies have successfully elicited a wide range of emotional responses by stimulating the hand region. The purpose of the current study was to test whether tactile stimuli applied to the torso could elicit similar emotional responses. To this end, we created 45 custom vibrotactile patterns that were presented through a vibrotactile vest to the front, back, and both sides of the torso. The patterns covered a wide range of physical variables such as amplitude, trajectory, and continuity. In an exploratory experiment, participants rated the arousal and valence of these patterns. Emotional responses differed between the patterns, and detailed analyses suggested that vibration amplitude and intensity where these vibrations were applied influenced both valence and arousal judgments. In a follow-up experiment, we systematically varied the amplitude and location of the vibrations. Our results showed that lower amplitudes were less arousing and more pleasant than higher amplitudes. Similarly, vibrations to the back torso were less arousing and more pleasant than those applied to the front or both sides of the torso, which can be explained by the lower sensitivity on the back. Taken together, we suggest that perceived intensity partially explains the relationship between the emotionality of vibration patterns on the torso.

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.