Shomeek Mukhopadhyay, Duke University


Dynamics of Circular Contact Lines

Abstract:  The classical problem of rotating fluids has been studied for more than 300 years, starting with Newton's classic "bucket" thought experiment in which the free surface of the liquid assumes a parabolic profile. In a modern twist to this experiment we use a perfectly wetting fluid (PDMS oil) on an oxidized silicon wafer and spin the "post-modern" bucket at high enough speeds such that the parabolic profile( which neglects viscosity and surface tension) is negative. It is seen that the receding contact line is not only exceedingly stable to perturbations but the central part where the parabolic profile is negative never dewets but asymptotically goes to a very thin film state (height ~200nm). We also add a radial temperature gradient to the experiment , unlike all the studies to date which have concentrated on vertical temperature gradient. Now the radial Marangoni force competes with centrifugal and surface tension forces to create interesting patterns and different scaling laws for the receding thin film. One can also use the radial temperature gradient and centrifugal forces to study the fingering instability of small drops . Using interferometric techniques one can image the two dimensional flow profile in detail at the onset of the instability. We have also done some preliminary studies on liquid crystal drops through a fast quench and looked at the nucleation of defects. Most of the system can be modeled by the lubrication equation and some preliminary numerical work will be presented.

Joint work with: Bob Behringer (Duke Physics) and Tom Witelski (Department of Mathematics, Oxford University and Department of Mathematics, Duke University)