Soft matter physics

Stochastic equilibrium/non-equilibrium thermodynamics, and microrheology

Microrheology of simple and complex fluids

We are working in the lab to develop micro-rheology techniques which are essential to probe local or bulk rheological properties of a fluid. The main advantage of micro-rheology is that it requires microliters of sample and has the ability to probe local rheological properties over a broad frequency spectrum. Using optical tweezer based micro-rheology, one can study even the environment of a cell. In our first work in this field, we have shown that the coupled dynamics of two trapped bead interacting hydrodynamically shows motional resonance. The resonance frequency is a function of the viscosity of the fluidic environment in which these two particles are embedded. Thus, by tracking this resonance frequency one can infer the viscosity of the fluid, which we later demonstrated. The resonance frequency can be tracked from the amplitude peak or from the zero crossing phase. We have shown that the measurement from the phase is much more sensitive and accurate than the measurement from the amplitude. In this work, we used a self-developed Matlab code to calculate amplitude and phase from the particle motion which is attached here. Further, we are trying to extend this technique to a viscoelastic fluid. We have shown theoretically that by solving the Stokes-Oldroyd B equations in the linear regime, the frequency dependent viscosity of a viscoelastic fluid is a function of the polymer time constant, the polymer contribution of viscosity and the solvent viscosity. We are now working on the measurement techniques of these parameters of a viscoelastic fluid. Additionally, we are attempting to understand the non-linear behaviour of viscoelastic fluids so that we can develop methods to measure the corresponding parameters. In future, these techniques can be effective tools to understand the rheological properties of the cellular environment. 

Hydrodynamics in the micro-scale

Probing hydrodynamic interactions using Optical Tweezers 

Understanding hydrodynamic would actually be a significant step towards understanding life itself - since cells, the basis of life - have a fluidic environment inside them, and thus the key to understand intercellular processes would be to understand the basics of hydrodynamic interactions. OT provides an excellent opportunity towards this, since micro-particles are trapped inside a fluid, and there are several detection techniques to quantify the dynamics of the trapped particles. However, this particular area of research in OT has taken off only in the last decade or so, and while it is quite a hot area now, there is also plenty of scope to perform new experiments and theory. 

We have started off in this direction only recently, and are currently studying interactions between a single trapped spherical particle with its fluidic environment, as well as that between two trapped particles in order to properly calibrate our system. Recently we have verified the fluctuation-dissipation relation in viscously coupled oscillators. A pair of trapped particles were probed in a viscous medium. One of the traps was driven sinusoidally and the response to the other was observed. After analyzing the frequency dependent phase and amplitude, we obtained a peak in the response function at a certain driving frequency alike a resonance in coupled harmonic oscillators, though the entire system is overdamped (see our Phys. Rev. E (Rapid Comm.) paper). 

Observing surface effects using Optical Tweezers 

An adhesive force typically arises due to the interaction of mesoscopic particles with a small separation between them. This is the London-Van der Waals (LVdW) force, and it acts between two particles as a function of radii, separation distance, and also on the inherent properties of the surrounding medium. In this regime, the medium viscosity is also no longer constant but varies as a function of separation between the surfaces. To study the phenomenon, we trapped a particle in the vicinity of a surface in a viscous medium and applied a sinusoidal perturbation to the particle. Thus, the particle acted like a probe to measure the effects of the force which was performed by determining the effective amplitude response of the particle. Indeed, this method gives a rather enhanced S/N ratio which is very effective in measuring the LVdW force, which can be rather minute at separations of several tens of nanometers.