Surfactants and fundamental flow fields at interfaces *

Interfacially trapped colloids generate long-ranged flows with signatures that reveal the compressibility and the rheology of interfaces. Surfactants, even in a modest concentration, significantly alter these mechanics. We dissect interfacial flows around Brownian colloids to provide insights essential to understanding these effects in dilute surfactant systems. Flow fields, measured by correlated displacement velocimetry, are decomposed into interfacial hydrodynamic multipoles, including force monopole and dipole flows. Prior work has shown that trace surfactant impurities render the interface incompressible. Here we address a Gibbs monolayer of surfactant undergoing a surface phase transition. The flow structure shows that the interface is incompressible for scant surfactant near the gaseous state, but is compressible in two phase coexistence between the gaseous and liquid-expanded state, and incompressible again in the liquid expanded state.  We also realize a non-trivial consequence of the surfactant concentration on the drag imposed on interfacially trapped colloids pointing towards hidden influences of surfactants on the mechanics of interfaces.