Evaporative Deposition of Bacteria from a Sessile Drop: Effects of Suspension Aging
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Evaporative Deposition of Bacteria from a Sessile Drop: Effects of Suspension Aging Kyle Baughman1, Raina M. Maier2 and Joan E. Curry2 1 Department of Materials Science and Engineering, The University of Arizona, Tucson, AZ 85721 2
Department of Soil, Water and Environmental Science, The University of Arizona, Tucson, AZ
85721 ABSTRACT In this work, we report on the evaporative deposition of bacteria from a drying aqueous drop on mica as a function of the bacterial suspension age. For sufficiently aged bacterial suspensions, residues are small and more filled-in than residues formed from fresh suspensions on similarly aged mica. In addition, the interior deposition pattern transitions from a cellular film characteristic of fresh suspensions to a cracked carpet pattern for aged suspensions. Suspension aging related changes in the residues are attributed to accumulation of organic materials such as DNA, RNA, proteins, and other bacterial components in the suspension. The aging process is also observed to be at least partially dependent on ventilation of the suspension during aging. INTRODUCTION With an aim to develop a quick, inexpensive way to pattern bacteria, we have studied sessile drops of bacterial suspensions. The drying of drops on surfaces (sessile drops) has been studied in a wide variety of non-living systems [1, 2]. A fully satisfactory general model able to explain the drying of these drops and the patterns they leave behind remains absent [1, 3]. Deposition of particles generally falls into two categories, particles deposited at the three-phase contact line which was discussed by Deegan [4] and particles deposited within the area encompassed by the three phase contact line [1, 3]. Drying sessile drops are widely known for causing suspended solids to aggregate at the edge of the drop. Deegan et al. [4] attributed this effect, called the coffee drop ring stain, to a combination of capillary forces at the three phase contact line, evaporation, and a phenomenon called pinning. Pinning is when the three phase contact line is not allowed to recede continuously during the evaporation process. Deegan et al. and others have written about various interior patterning of non-biological particles (e.g. hydroxyapatite [2] and polystyrene [1, 2]). Deegan et al. also showed that particle deposition can be varied by introducing sodium chloride which presumably alters the particle-particle and particle-surface interactions [3]. In recent work [5], Baughman showed that drops of freshly prepared and washed bacterial suspensions deposited on mica leave residues on evaporation that depend on the amount of time the mica has been exposed to filtered laboratory air. At short exposure times, residues are large and the interior is characterized by a series of depinning rings which are similar to the “coffee ring stains” reported by Deegan [4]. At long exposure times, residues are small and completely covered with cellular films. By simply controlling the buildup of atmospheric components on the surface, one can man
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