Surface proximity effect in sedimentation investigated by digital holographic microscopy.

In this paper, we measure the proximity effect of a flat wall on the sedimentation of colloidal particles by the use of digital holographic microscopy (DHM). DHM provides quantitative phase contrast and three-dimensional (3D) imaging in arbitrary time scales that are suitable to investigate various phenomena, including dynamic behavior of colloids. 3D information is obtained by post-processing of the recorded digital holograms through the angular spectrum propagation approach for numerical reconstruction. We have measured the trajectory and velocity of spherical colloids in various distances from a flat wall while they sediment under gravity. The results are in agreement with theoretical studies of Goldman et al. [Chem. Eng. Sci.22, 637 (1967)CESCAC0009-250910.1016/0009-2509(67)80047-2]. We observed an increase in the drag force due to the proximity to the flat wall, which was effective for distances up to several particle sizes. The presented method is capable of being used for different applications in study of colloidal dynamics where 3D information in real time is required.