Prof Ramin Golestanian

Abstract:

We present an experimental study of the coiling instability of a liquid "rope" falling on a solid surface. Coiling can occur in three different regimes--viscous, gravitational, or inertial--depending on the fluid viscosity and density, the fall height, and the flow rate. The competition among the different forces causes the coiling frequency first to decrease and subsequently to increase with increasing height. We also observe an oscillation between two coiling states in the gravitational-to-inertial transitional range, reflecting the multivaluedness of the dependence of coiling frequency on fall height. The data can be rescaled in a universal way, and agree very well with numerically predicted coiling frequencies.

Abstract:

In a fluctuating medium, whether of quantum, thermal, or non-thermal origins, an interaction is induced between external objects that modify the fluctuations. These interactions can appear in a vast variety of systems, leading to a plethora of interesting phenomena. Notable examples of these include: 1. like-charge attraction in the presence of multivalent counterions; 2. Ludwig-Soret effect in charged colloids; 3. mass renormalization of moving defects in a phononic background and moving metallic objects in electromagnetic quantum vacuum; 4. dissipation due to motion-induced radiation. Another related class of problems corresponds to stirring the media by dynamic deformations of the embedded bodies and benefiting from the back-reaction of the stirred media for force generation, such as force generation in swimming. The fluctuation-induced forces are statistical in nature, and this could make their measurements very difficult, because the actual value of the force might deviate most of the time from the predicted average value.

Abstract:

The dynamics of the deformations of a moving contact line on a disordered substrate are formulated, taking a proper account of the various interfacial forces as well as the dissipation mechanisms. Prompted by the results from dynamical renormalization group calculations, it is suggested that the coating transition in contact lines receding at relatively high velocities can be understood as a roughening transition in the contact line. A phase diagram is proposed for the system in which the phase boundaries corresponding to the coating transition and the pinning transition meet at a junction point, and suggest that for sufficiently strong disorder a receding contact line will leave a Landau-Levich film immediately after de-pinning.

Abstract:

The buckling of multi-walled carbon nanotubes and rods was discussed. It was shown that electrostatic interactions exert a significant effect on the buckling instability of a rod. The threshold value of the compressional force needed to induce buckling was found to be independent of rod length for long charged rods. The critical buckling force crosses over from the classic inverse-square length dependence to asymptotic length-independent form with increasing rod length, in the case of rods of intermediate length. It was suggested that this effect leads to the possibility of electromechanical stiffening of nanotubes, which would allow relatively long segments of them to be used as atomic force probes.