Anushka Jha

Active Microrheology of Soft Materials Using Optical Tweezers

In this work, we have carried out active microrheology of three different soft materials- aqueous solutions of un-crosslinked polyacrylamide with high molecular weight (5000kDa), aqueous suspensions of crosslinked polymeric gel- Carbopol 940 and aqueous suspensions of hectorite clay- Laponite XLG using femtosecond laser tweezers. Optical trapping has been carried out using two methods- passive optical trapping in which the probe particles are observed under a stationary trap and active optical trapping in which the stage is given a periodic motion in the x-direction. The advantage of active optical trapping is that real-time calibration of the set-up is possible along with measurement of the properties of the system. PAm and Carbopol 940 are studied as examples of systems in equilibrium. Laponite has been studied as it is known to undergo time-dependent changes in its physical properties- also known as aging. Viscosity data has been obtained for these materials using probe particles of different sizes to gain insight into the microstructure of the soft materials and verify the validity of the methods used. The dependence of viscosity and time-scales of particle fluctuation at different length scales have been obtained as a function of concentration. The microrheological viscosity obtained has been compared with data obtained from bulk rheology experiments to demonstrate the applications as well as limitations of the given microrheology techniques in probing soft materials with complex microstructural arrangements.