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42858-AC7
Optically Driven Nonlinear Microrheology
Our second year has been very productive, and we have taken our nonlinear microrheology experiments beyond steady-shear measurements of poly(ethylene oxide) solutions that we found in the first year. After showing that it is possible to measure a shear-thinning viscosity using optically driven rotational microrheology of birefringent wax microdisks, we have been primarily preoccupied with determining nonlinear responses of soft materials that are more elastic. In these highly elastic complex fluids, the disks do not simply spin freely when an optical torque is applied; instead, the elastic resistance of the material is much more important. In particular, we have focused on determining the localized microscopic yield stress of an aqueous gelatin network.
By dispersing wax microdisks in gelatin solutions at very dilute concentrations and allowing the gelatin to form an elastic network, we create a soft complex material that is nearly transparent to laser light, thereby facilitating tweezing of the microdisks. In order to obtain adequate torques that could potentially cause the gelatin network to yield and flow around a microdisk, we have improved our laser tweezer apparatus by incorporating a krypton ion laser that can attain powers of up to 1.5 Watt. Using this laser, we have conducted systematic measurements of the angular displacement of the wax microdisk as a function of laser power. Using a known calibration of the torque on a disk from steady rotation in water, we can reasonably estimate a stress on the disk using known drag factors associated with the disk's geometry. When a small stress is applied abruptly by causing a shutter to open, the disk rotates to a certain angle and then remains at that angle until the light is blocked again. Typically, the recovery is very good after the optical stress is removed, and the disk rotates back nearly to its initial position. At the largest stresses, however, the disk continues to slowly rotate while the stress is held constant over long times, indicating that there is a viscous creep characteristic of yielding behavior. We believe that this is the first clear rotational step-stress measurement that shows the onset of non-linear yielding behavior. Overall, we think that this microrheological approach will be useful for extracting local non-linear microscopic rheology of complex fluids. We have prepared a manuscript of our non-linear measurements of optically driven rotational microrheology in which we acknowledge ACS PRF, and it is nearly ready for submission to Phys. Rev. E.
In addition to making these advances, during the second year, we have also constructed a dual beam laser tweezers apparatus that is based on two smaller helium-neon lasers (20 mW). We are starting to use these dual rotational laser tweezers to perform two-particle rotational microrheology experiments. This work is more complicated than the single-disk approaches, so it is only starting. We are excited about continuing this work; it would be wonderful if ACS-PRF would consider extending this AC grant to a third year that we requested in our original proposal.
In a collaboration related to microrheology, the PI has co-authored three papers with researchers from the medical school at UCLA. Portions of the experimental apparatus, techniques, and insights that we gained through work on this grant carried over into this research and the resulting publications, so we gratefully acknowledge ACS PRF for providing partial support.
Overall, we really appreciate the support and funding from ACS PRF, since it has resulted in a new understanding of nonlinear microrheology and also enhanced the education of a UCLA graduate student.
