Hydrodynamic coupling in driven colloid.

摘要

We demonstrate that the onset of the convective stratification observed in a plethora of colloidal suspensions is initiated by non-convective nonlinear concentration waves known as Burgers shocks resulting from the hydrodynamic coupling of rising particles. These observations highlight the need for new experimental tools with which to study colloidal hydrodynamics, particularly under the influence of external forces. In meeting this need, we have developed an extremely general category of new experimental techniques which we call dynamic holographic optical tweezers (DHOTs). This revolutionary class of micromanipulation techniques can create arbitrary configurations of optical traps in three dimensions as well as mixed arrays of unconventional optical traps such as optical vortices, optical Bessel beams and optical line tweezers. We also introduce a novel optical trap called the modulated optical vortex. We describe algorithms for calculating the necessary holograms for such optical trapping configurations and then describe the dynamic holographic optical tweezer apparatus used to investigate the intensity structure and orbital angular momentum content of the optical vortex. These investigations reveal that the optical vortex scales qualitatively differently from predictions, and this discrepancy is explained on the basis of scalar diffraction theory. Furthermore, the measurements of the optical driving of a single particle are consistent with predictions that each photon in a helical beam carries lħ orbital angular momentum. Finally, we observe that the motion of a particle on an optical vortex created with a pixellated spatial light modulator (SLM) provides evidence for a novel optical ratchet potential around the optical vortex's circumference.