Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Near-surface dynamics of particle-laden water and semidilute polymer solutions: Diffusion, nonlinear rheology, and the hydrodynamic boundary condition
Nov 8, 2019
10:15 - 11:15
Joshua D. McGraw
- ESPCI, Paris, France
Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
In near-surface flows, interfaces play a major role by imposing, often, a no-slip or a weak slip boundary condition. Such a condition greatly reduces the fluid velocity compared to the central part of a channel. With total internal reflection fluorescence (TIRF), a flow is illuminated with an evanescent field decaying over a few hundred nanometers into the channel; this decay then allows a determination of nanoparticle altitudes. Combined with particle tracking, experimental determination of the velocity profile and local velocity distributions in three dimensions are possible. Here we present a detailed look at the statistics of near-surface particle motions in pressure-driven flows for two systems: pure water and semidilute polymer solutions. In both cases, the near-wall velocities are small such that diffusion is important compared to advection.
For water the distribution of displacements in the invariant flow direction is Gaussian as for normal diffusion. Significant anomalies are however observed for both of the other spatial dimensions. Combining experiments and simulations, we disentangle contributions from so-called Taylor-Aris dispersion near the wall, nanoparticle polydispersity and the optical measurement system. This description of TIRF allows for the study of many Brownian motion problems, such as near-surface polymer solution dynamics. Indeed, the near-surface dynamics of polymer solutions challenge both experimental and theoretical efforts — especially in the case of semidilute solutions for which the chains overlap but the solvent still plays an important role. Experiments using hydrogenated polyacrylamide at different volume fractions close to and above the overlap concentration are done in the same chips as for the water experiments. In contrast to Newtonian fluid behaviour, the shear-rate/pressure drop relation is non-linear for the polymer solution flows, suggesting nanometrically-resolved, shear-thinning effects, accompanied with a non-trivial hydrodynamic boundary condition. The diffusive motion of the tracer particles is also distinguished from that of the water experiments, and such motions are detailed here. These results set the basis for a study of near-wall hydrodynamic flow and diffusion in complex fluids.