Random motion with interfacial contact: Driven diffusion vis-à-vis mechanical activation

• Published in: The European physical journal. E, Soft matter and biological physics Soft matter and biological physics, 2012-08, Vol.35 (8), p.67-67, Article 67
• Main Authors: Goohpattader, P. S., Chaudhury, M. K.
• Format: Article
• Language: English
• Subjects: Biological and Medical Physics; Biophysics; Complex Fluids and Microfluidics; Complex Systems; Condensed matter: structure, mechanical and thermal properties; Diffusion; interface formation; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Mechanical contact (friction...); Nanotechnology; Physics; Physics and Astronomy; Polymer Sciences; Regular Article; Soft and Granular Matter; Solid mechanics; Solid surfaces and solid-solid interfaces; Structural and continuum mechanics; Surfaces and Interfaces; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Thin Films
• ISSN: 1292-8941; 1292-895X
• DOI: 10.1140/epje/i2012-12067-0

Rolling of a small sphere on a patterned support of an elastomer is governed by a non-linear friction. No motion occurs when the external field is weaker than the frictional resistance. However, with the intervention of an external noise, a viscous friction like behavior emerges; thus the sphere rolls with a uniform drift velocity that is proportional to the applied field. At a very low noise strength, the sphere exhibits a stick-slip behavior with motion occurring always along the bias. With the increase in the noise strength, the sphere exhibits a diffusive drift accompanied with forward and backward displacements. During this stage of driven diffusive motion, the ratio of the integrated probabilities of the negative-to-positive work fluctuations decreases monotonically with the time of observation, from which a temperature like intensive parameter can be estimated. This parameter conforms to Einstein’s ratio of diffusivity and mobility that increases almost linearly, even though the diffusivity increases super-linearly, with the strength of the noise. A new barrier crossing experiment is introduced that can be performed either with a hard ( e.g. a steel ball) or with a soft ( e.g. a water drop) sphere in contact with a periodically undulated substrate. The frequency of barrier crossing follows a transition state equation allowing a direct estimation of the effective temperature. These experiments as well as certain numerical simulations suggest that the effective temperature of a system controlled by a non-linear friction may not have a unique value.