Prof Ramin Golestanian

Abstract:

The microscopic mechanism for thermodiffusion or the Ludwig-Soret effect is investigated for a single charged colloidal particle. For the specific example of a charged permeable membrane, the different forces arising from the interplay between electrostatic interaction and entropy are identified and their magnitudes are calculated in both no-salt and salt-saturated limits. The competition between these forces, which is controlled by the salt density, is shown to decide the direction of motion for the colloids. © 2005 IOP Publishing Ltd.

Abstract:

In a fluctuating medium of quantum, thermal, or non-thermal origin, 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 EM quantum vacuum, and (4) dissipation due to motion-induced radiation. The fluctuation-induced forces are statistical in nature, and this could make their measurement very difficult, because the actual value of the force might deviate most of the time from the predicted average value. © Indian Academy of Sciences.

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.