Hydrodynamics of Brownian colloids in soluble surfactant layers

The Brownian motion of passive colloids embedded on fluid interfaces is governed by fluctuation dissipation theorem. This enables the measurement of interfacial dynamics by the tracking of single colloidal particles and measuring their correlated behaviors.

We study the flow fields generated around the Brownian particles at the air-aqueous interface of  surfactant solution using correlated displacement velocimetry. The leading order flow around the particles on or near the interface is the interfacial Stokeslet. The form of the Stokeslet can reflect the interfacial stress state, e.g., whether the interface can be described by a uniform surface tension or whether Marangoni stresses are present that can render the interface incompressible. In our prior work, we observed the form of an incompressible interfacial Stokeslet around Brownian particles, even though we took pains to avoid surfactant contamination.

In this study, we deliberately added two soluble surfactants in the system that allow us to probe the effect of surfactants on interfacial mechanics. In one case, surfactants that undergo first-order surface phase transitions can result in decoupling of surface tension from surface concentration. In another case, soluble surfactant with rapid exchange between the interface and the bulk can decrease surface concentration difference around the colloids. In both cases, flow around the Brownian particles is observed to deviate significantly from the incompressible Stokeslet.