Experiment and modelling of texture and sliding direction dependence on finger friction behavior

Humans rely on their fingers to sense and interact with external environment.Understanding the tribological behavior between finger skin and object surface is crucial for various fields,including tactile perception,product appearance design,and electronic skin research.Quantitatively describing finger frictional behavior is always challenging,given the complex structure of the finger.In this study,the texture and sliding direction dependence of finger skin friction was quantified based on explicit mathematic models.The proposed double-layer model of finger skin effectively described the nonlinear elastic response of skin and predicted the scaling-law of effective elastic modulus with contact radius.Additionally,the skin friction model on textured surface considering adhesion and deformation factors was established.It revealed that adhesive term dominated finger friction behavior in daily life,and suggested that object texture size mainly influenced friction-induced vibrations rather than the average friction force.Combined with digital image correlation(DIC)technique,the effect of sliding direction on finger friction was analyzed.It was found that the anisotropy in finger friction was governed by the finger’s ratchet pawl structure,which also contributes to enhanced stick-slip vibrations in the distal sliding direction.The proposed friction models can offer valuable insights into the underlying mechanism of skin friction under various operating conditions,and can provide quantitative guidance for effectively encoding friction into haptics.

Design of bidirectional frictional behaviour for tactile contact using ellipsoidal asperity micro-textures

Tactile perception and friction can be modified by producing a deterministic surface topography.Change of surface feature arrangement and texture symmetry can produce an anisotropic frictional behaviour.It is generally achieved through skin hysteresis by promoting its deformation.This work investigates whether a bidirectional friction can be created with microscale ellipsoidal asperity textures,thus relying on the adhesive component of friction.For this purpose,four textured samples with various asperity dimensions were moulded with a silicone rubber having an elastic modulus comparable to that of the skin.Coefficient of friction measurements were conducted in-vivo in two sliding directions with a range of normal loads up to 4 N.Finite element method(FEM)was used to study elastic deformation effects,explain the observed friction difference,and predict surface material influence.Measurements performed perpendicular to the asperity major radii showed consistently higher friction coefficients than that during parallel sliding.For the larger asperity dimensions,a change of the sliding direction increased friction up to a factor of 2.The numerical analysis showed that this effect is mostly related to elastic asperity deflection.Bidirectional friction differences can be further controlled by asperity dimensions,spacing,and material properties.