(9f) Displacement Fields Around Thermally Forced and Active Colloids at Interfaces

Authors: 
Molaei, M., University of Pennsylvania
Deng, J., University of Pennsylvania
Chisholm, N. G., University of Pennsylvania
Crocker, J. C., University of Pennsylvania
Stebe, K., University of Pennsylvania
While theory provides guidance on expected flows around colloids at interfaces, there is scant direct experimental measurement to compare to these predictions, hampering our understanding of these systems. This is so for colloids that move under applied forces, or colloids that move under their own action.

Here, we simultaneously measure the mechanics of the interface and discern displacement fields in two settings. First, we study motion induced by Brownian displacements of colloids on their neighbors. We measure the extremely weak correlated motion induced by the thermal displacement of one colloid one its neighboring colloids as a function of distance and angular position in the interface. We use this information to generate a map of the displacement field that occurs over different lag times and different interfacial mechanics conditions. Thereafter, we address the motion of active colloids (bacteria) trapped on or near an oil-water interface. We analyze the flow fields generated by active colloids at interfaces by imaging displacements owing to trapped bacteria. We perform these measurements for several swimming behaviors including pushers, pullers, and lassoers.

These measurements are compared to calculated displacement fields for hydrodynamic modes permitted in interfacial layers as a function of interface shear and dilatational viscoelasticity and compressibility and hydrodynamic coupling with bulk fluid.