Particle driven in a crystalline colloidal monolayer

Probes driven in interfaces displace passive colloidal tracers to generate displacement fields which bear the signature of the degree of organization of the colloidal tracer monolayer. We study the displacement fields around driven ferromagnetic probes particles in monolayers of PS colloidal particles at oil-water interfaces in gaseous, liquid, and crystalline states, increasing the importance of many body effects as order in the monolayer increases. Using correlated displacement velocimetry, we measure the flow field owing to thermal motion of the PS particles absent the driven particles. In the gaseous state, the flow field agrees with the form expected for a Stokeslet in an incompressible interface. In more organized states, the flow field reveals signatures of the coupling between the hydrodynamic and electrostatic interactions. We also characterize the motion around the driven magnetic probe in the ordered colloidal monolayer. Long range motion of the monolayer is observed; this manifestation of many-body electrostatic interactions is discussed in terms of spatial order and vibrational modes. Understanding the coupling between the hydrodynamic and electrostatic interactions allow us design active interfaces that fulfill different needs on interfacial functionality.