61st Annual Report on Research 2016

The goal of our proposed project is to design novel colloidal suspensions with desired non-Newtonian flow behaviors. In particularly, we are interested in controlling shear thinning and thickening behaviors of suspension flows. We proposed to achieve such a control by tuning inter-particle attractions, particle shapes and confinement of system. The support of ACS Petroleum Research Fund in the first year allows us to study the effect of confinement on the dynamics and rheology of colloidal suspensions (see our report from the last year and our publication in PRL 2016). In the second year (09/01/2015-09/01/2016), we have investigated the effect of particle activity on suspensions’ dynamics and rheology. Particle activity dramatically changes interparticle interactions and modifies the rheological behaviors of the suspensions. We have made important progress in our study during this period, which I shall briefly describe below.

We have studied the dynamics and rheology of active suspensions using our ultra-fast confocal rheoscope. Active suspensions (or active fluids) are a novel class of non-equilibrium suspensions, which compose of large number of self-propelled units that convert the ambient or internal free energy and maintain non-equilibrium steady states at local scales. Due to this distinct feature, active suspensions show unique properties different from conventional colloidal suspensions. Particularly, they show interesting rheological behaviors, ideal for our research goal for designing non-Newtonian fluids with controllable flow behaviors. Theoretical studies have shown that one type of active suspensions—pusher suspensions—can significantly lower the bulk viscosity of active suspensions, to such an extent that active suspensions can have a lower viscosity than the suspending fluids. The other type of active suspensions—puller suspensions—can enhance the viscosity of suspending fluids. As such, by controlling the activity of suspended particles, one will be able to control shear thinning or shear thickening behaviors of suspensions. Our research aims to realize such a possibility.

(1) In our second year, we first characterized the dynamics and microrheology of the premier examples of pusher and puller active suspensions, i.e., bacterial suspensions of E. coli and algal suspensions of C. reinhardtii. The two suspensions show shear thinning and thickening behaviors, respectively. To quantify the dynamics of the two suspensions, we inserted micron-sized ellipsoids as our tracer particles in these suspensions (Fig. 1a and b). Similar to microrheology for equilibrium suspensions, we revealed the unusual dynamics and rheology of active suspensions by monitoring the diffusion of the tracer particles. Different from previous studies, where the diffusion of spherical tracers was studied, we used ellipsoids of asymmetric shapes in our study. We showed that, due to the additional rotational degree of freedom that is absent for spherical tracers, the dynamics of asymmetric tracers show a profound difference in shear thinning bacterial suspensions and shear thickening algal suspensions. Although the lab frame translation and rotation of ellipsoids are enhanced in both types of suspensions, similar to spherical tracers, the anisotropic diffusion in the body frame of ellipsoids shows opposite trends in the two classes of active fluids. An ellipsoid diffuses fastest along its major axis when immersed in pullers, whereas it diffuses slowest along the major axis in pushers (Fig. 1c). This striking difference can be qualitatively explained using a simple hydrodynamic model. Such a difference allows us to detect the class of active particles, which are crucial for determining the rheological flow behaviors of active suspensions. Two papers based on the work were either published or accepted recently in Physical Review Letters and Physical Review E.

(2) We also used our ultra-fast confocal rheoscope to study the flow profile of sheared active suspensions (Fig. 2a). Rather than increasing the viscosity of the suspending fluid like normal colloidal suspensions, pusher-type active particles can self-organize into collective flows under shear, turning active suspensions into “superfluids” with zero apparent viscosity. Although the existence of the active superfluids has been demonstrated in bulk rheology measurements, little is known about the microscopic dynamics of such an exotic phase. By combining sensitive rheology measurements with high-speed confocal microscopy, we study the detailed 3D dynamics of concentrated bacterial suspensions under oscillatory shear. We found that sheared bacterial suspensions in the superfluidic phase at low shear rates exhibit velocity profiles with strong spatial heterogeneity, unexpected from the established rheological theory of active fluids. Our confocal rheoscope is composed of a parallel shear cell. Near the bottom moving shear plate, strong shear gradients are developed in sheared active suspensions. However, deep inside sheared suspensions, the shear gradient completely vanishes in the sheared suspensions (Fig. 2c), which gives rise to the unusual superfluidic behaviors. Our experiments reveal a profound influence of shear flows on the activity of particles and provide new insights to the origin of the unique flow behaviors of active suspensions. We are currently writing a paper based on this result.

The support of ACS Petroleum Research Fund greatly strengthens our research efforts and deepens our understanding of the rheology and dynamics of colloidal suspensions, particularly, active suspensions. The projects supported by the PRF grant have resulted in three papers published or accepted over the last two years. Another paper is under preparation. Moreover, using the preliminary results obtained from the PRF-supported projects, my proposals have won the NSF Career Award, DARPA Young Faculty Award, and Packard Fellowship.

Figure 1: Dynamics of micron-sized ellipsoids in active suspensions. (a) A schematic of our experimental setup. (b) Trajectories of an ellipsoidal tracer and algae. Scale bar = 20 μm. (c) Anisotropic diffusion of ellipsoids, D_a/D_b, increases with algal concentrations, whereas it decreases with the concentration of active particles in pusher-type active fluids.

Figure 2: Velocity profiles of sheared active suspensions. (a) A schematic of our confocal rheoscope. (b) Displacements of active suspensions of E. coli under oscillatory shear as a function of time at different heights y above the shear plate at y = 0 μm. The total gap size H = 30 μm. The applied shear period is T = 10 s and the shear amplitude is A₀ = 100 μm (b) and 20 μm (c).