Interfacial colloidal assembly guided by optical tweezers and tuned via surface charge.

Hypothesis: The size, shape and dynamics of assemblies of colloidal particles optically-trapped at an air-water interface can be tuned by controlling the optical potential, particle concentration, surface charge density and wettability of the particles and the surface tension of the solution.

Experiments: The assembly dynamics of different colloidal particle types (silica, polystyrene and carboxyl coated polystyrene particles) at an air-water interface in an optical potential were systematically explored allowing the effect of surface charge on assembly dynamics to be investigated. Additionally, the pH of the solutions were varied in order to modulate surface charge in a controllable fashion. The effect of surface tension on these assemblies was also explored by reducing the surface tension of the supporting solution by mixing ethanol with water.

Findings: Silica, polystyrene and carboxyl coated polystyrene particles showed distinct assembly behaviours at the air-water interface that could be rationalised taking into account changes in surface charge (which in addition to being different between the particles could be modified systematically by changing the solution pH). Additionally, this is the first report showing that wettability of the colloidal particles and the surface tension of the solution are critical in determining the resulting assembly at the solution surface.