Project VI – Confined soft condensed matter exposed to shear deformations

Contributor: M. Schoen

If soft condensed matter is confined by solid substrates to spaces of nanoscopic dimensions many of its properties are altered compared with those of a corresponding bulk phase under identical thermodynamic conditions. One of the most striking examples is the resistance of a confined fluid film against shear deformation. The response of such a film to a shear deformation resembles that of ordinary bulk solids, that is the shear stress depends linearly on the strain (Hookean response) if the applied strain is sufficiently small; at larger strains the response becomes more plastic eventually causing a yield point to exist where the shear stress attains a maximum (i.e., where the associated shear modulus vanishes). This behavior of confined films consisting of small organic molecules is frequently investigated experimentally by means of the surface forces apparatus (SFA). SFA experiments usually deal with low shear rates so that misalignment of the confining substrates is vanishingly slow on the typical length and time scales of relaxation processes in the fluid film. This permits to view shearing as a quasistatic process and apply equilibrium statistical physics to investigate structure, thermomechanical properties (e.g., the shear stress), and phase behavior theoretically. Within this theoretical framework we plan to employ Monte Carlo simulations in specialized mixed stress-strain ensembles, density functional, and Landau theory to study the behavior of various models of confined films exposed to shear deformation