Julia Yeomans OBE FRS

Abstract:

We model an infinitely long liquid bridge confined between two plates chemically patterned by stripes of the same width and different contact angle, where the three-phase contact line runs, on average, perpendicular to the stripes. This allows us to study the corrugation of a contact line in the absence of pinning. We find that, if the spacing between the plates is large compared to the length scale of the surface patterning, the cosine of the macroscopic contact angle corresponds to an average of cosines of the intrinsic angles of the stripes, as predicted by the Cassie equation. If, however, the spacing becomes on the order of the length scale of the pattern, there is a sharp crossover to a regime where the macroscopic contact angle varies between the intrinsic contact angle of each stripe, as predicted by the local Young equation. The results are obtained using two numerical methods, lattice Boltzmann (a diffuse interface approach) and Surface Evolver (a sharp interface approach), thus giving a direct comparison of two popular numerical approaches to calculating drop shapes when applied to a nontrivial contact line problem. We find that the two methods give consistent results if we take into account a line tension in the free energy. In the lattice Boltzmann approach, the line tension arises from discretization effects at the diffuse three phase contact line. © 2009 American Chemical Society.

Abstract:

We investigate the effects of polymer chains and nanoparticles on the deformation of a droplet in shear and extensional flow using computational modeling that accounts for both the solid and fluid phases explicitly. We show that under shear flow, both the nanoparticles and the encapsulated polymers reduce the shear-induced deformation of the droplet at intermediate capillary numbers. At high capillary numbers, however, long polymer chains can induce the breakup of the droplet. We find that the latter behavior is dependent on the nature of the imposed flow. Specifically, under extensional flow, long polymers inhibit the droplet breakup and reduce deformation. Overall, the findings provide guidelines for tailoring the stability of filled droplets under an imposed flow, and thus, the results can provide useful design rules in a range of technological applications. © 2009 American Institute of Physics.

Abstract:

We describe in detail the hydrodynamics of a simple model of linked sphere swimmers. We calculate the asymptotic form of both the time averaged flow field generated by a single swimmer and the interactions between swimmers in a dilute suspension, showing how each depends on the parameters describing the swimmer and its swimming stroke. We emphasize the importance of time reversal symmetry in determining the far field flow around a swimmer and show that the interactions between swimmers are highly dependent on the relative phase of their swimming strokes.