Evaporative Deposition of Bacteria and Microspheres on Mica from a Sessile Drop: The Use of Surface Conditioning in a Laboratory Atmosphere to Control Drop Spreading and Particle Deposition Patterns

Evaporative deposition from a sessile drop is an appealing way to deposit materials on a surface due to the simplicity of the technique. In this work we deposit aqueous solutions of two types of colloidal particles, namely bacteria and microspheres, on mica. We show that by controlling the extent of initial drop spreading through subtle changes in surface conditioning caused by exposure to the laboratory atmosphere in a laminar flow hood it is possible to systematically vary the particle deposition patterns. On freshly cleaved mica the contact angle of water is < 5°. Drops of bacterial and microsphere solutions deposited on freshly cleaved mica spread to cover a large surface area. Drying occurs through pinning and depinning events leaving a series of colloidal particle rings. We found in our laboratory that the contact angle of water on mica exposed to a constant flow of filtered laboratory air in a laminar flow hood gradually increases with time. For drops of both bacterial and microsphere solutions there is a corresponding decrease in the extent of drop spreading with increasing exposure of the mica surface to laboratory air. This results in a profound change in the colloidal particle deposition pattern. Short exposures of minutes to hours are enough to decrease spreading and affect the resulting deposition pattern. For our longest mica surface exposure times (months to 1 year) the contact angle of water reaches values near 20°. Spreading of the bacterial and microsphere drops is substantially decreased. A portion of the colloidal particles are deposited in an outer deposition ring which marks the extent of drop spreading and the remainder of the particles are deposited in the drop interior as a honeycomb or cellular film. The fraction of the drop residue covered with the cellular film increases with particle concentration as well as the length of time the mica is exposed to the laboratory atmosphere. This work shows that evaporative deposition on mica is very sensitive to surface conditioning through atmospheric exposure and also suggests that particle deposition patterns can be tuned by small changes in drop spreading.