Evaporation-driven convective flows in suspensions of nonmotile bacteria

Abstract

We report a novel form of convection in suspensions of the bioluminiscent marine bacterium Photobacterium phosphoreum. Suspensions of these bacteria placed in a chamber open to the air create persistent luminiscent plumes most easily visible when observed in the dark. These flows are strikingly similar to the classical bioconvection pattern of aerotactic swimming bacteria, which create an unstable stratification by swimming upwards to an air-water interface, but they are a puzzle since the strain of P. phosphoreum used does not express flagella and therefore cannot swim. Systematic experimentation with suspensions of microspheres reveals that these flow patterns are driven not by the bacteria but by the accumulation of salt at the air-water interface due to evaporation of the culture medium; even at room temperature and humidity, and physiologically relevant salt concentrations, the rate of water evaporation is sufficient to drive convection patterns. A mathematical model is developed to understand the mechanism of plume formation, and linear stability analysis as well as numerical simulations were carried out to support the conclusions. While evaporationdriven convection has not been discussed extensively in the context of biological systems, these results suggest that the phenomenon may be relevant in other systems, particularly those using microorganisms of limited motility.

Evaporation-induced Rayleigh–Taylor instabilities in polymer solutions

“…The Goldstein group were the ones to discover the mobility mechanism described above. Using physiological (dilute) saline concentrations ranging between 0.1 and 1% they find that the onset times are '8 times longer for a 10 times more dilute suspension' [14]. This dependence of the onset time on concentration yields a power law exponent of −0.9, falling well within the limits of −1 and −2/3 developed in our paper.…”

“…As alcohol escapes from these 'evaporating cocktails', a water-rich surface layer develops, creating an unstable stratified system. Other examples include the geotaxis of microorganisms [13,14], the dissolution of minerals in freshwater [15] and mixing with seawater [16], granular media flows [17], and absorption of CO 2 in brine solutions for carbon capture and storage applications [18]. Mixing in the accidental painting technique [19] is an example of how artists exploit density differences between pigmented layers of paint to create interesting patterns.…”

Solutal convection induced by dissolution

“…Nevertheless, solute flows can occur without external forcing: buoyancy forces caused by differences in solute concentration can generate a convection flow. Literally analog to the thermal convection, where the density variations are caused by fluid thermal expansion, solutal convection can be triggered by evaporation [8,9] or freezing [10] of salt water, or dissolution. Solutal convection induced by dissolution is mostly studied in porous media especially in the context of CO 2 sequestration [11][12][13][14].…”

Mixing Salts and Poly(ethylene glycol) into Protein Solutions: The Effects of Diffusion across Semipermeable Membranes and of Convection

“…For a dense layer on top of a lighter layer, convection occurs when Ra S 10 3 . 28,31,39 Below that value there is only diffusion. If the dense layer is on the bottom, then it is stable and there is no convection.…”