RESEARCH
Active complex fluid systems
Active complex fluid systems such as living cells, in vitro assemblies of motors and filaments, and vibrated granular media differ fundamentally from conventional equilibrium media in that some of their components consume and dissipate energy, thereby creating a state that is far from equilibrium. An understanding of model active systems, even at a phenomenological level, provides insight about fundamental non-equilibrium statistical physics and, potentially, about the inner workings of biological systems. In this work, we use microrheology to measure the fluctuations and response of a model active system: a suspension of the bacterium E. coli. Our measurements of one- and two-point tracer particle correlations, when combined with the measured response function of a driven, optically trapped tracer, demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. A detailed theoretical model incorporating the interplay of coupled stress, orientation, and concentration fluctuations of the bacteria was derived and found to quantitatively account for our experimental observations, including the frequency dependent scaling of the power spectra. The significance of this work is that a basic analytic framework has been fruitfully applied to gain insight into a simple active system, thus establishing a promising approach to understand more complex active systems such as the interior of living cells.
 | Right: The measured power spectrum D(w) of tracer particles in wild-type and tumbler bacterial suspensions and in water alone. The power spectrum for an equilibrium suspension is flat, as observed for water, whereas the bacterial power spectra exhibit distinct deviations from equilibrium, depending on the microscopic swimming behavior. Left: Cartoon illustrating the swimming behaviors of wild-type and tumbler E. coli strains used in the experiments. |
Melting in Quasi-1D colloidal system
Some other interesting colloidal systems
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Upper left: Bright-field image of a single domain of 2D colloidal crystal with 5 defects. Upper right: colloidal gel. Lower left: Coexistence of liquid and crystalline phase. Lower right: Colloidal crystal in square lattice, along with the presence of grain boundaries.