Anisotropic Memory Effects in Confined Colloidal Diffusion
Understanding and controlling the transport of colloidal microcarriers through a fluid is one of the main challenges in cell biology, and related lab-on-a-chip approaches. With the miniaturization of such technologies, particularly microfluidics, the colloid's motion is increasingly confined, and the influence of boundaries becomes non-negligible. Any deviation from its well-understood free Brownian motion, will give us information on the particle's surroundings.
The motion of an optically trapped sphere constrained by the vicinity of a wall is investigated at times where hydrodynamic memory is significant. First, we quantify, in bulk, the influence of confinement arising from the trapping potential on the sphere's velocity autocorrelation function C(t). Next, we study the splitting of C(t) into the parallel directions and the perpendicular direction, when the sphere is approached towards a surface. Thereby, we monitor the crossover from a slow long-time tail with exponent -3/2, away from the wall, to a faster decay with exponent -5/2, due to the subtle interplay between hydrodynamic backflow and wall effects. Finally, we discuss the resulting asymmetric time-dependent diffusion coefficients.