Rearrangements at physical interfaces directing biology
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The movement of fluids has a significant impact on the biological world, from the transport of critical medications, to the shaping of cellular life. The presence of a fluid-fluid interface gives rise to regions where a fluid---and its contents---can be selectively transported or trapped, and where stresses from the rearranging interface can lead to damage or even death of nearby microorganisms. First, we examine the role of local displacement on network level transport. Multiphase fluid flow through small length-scale networks---such as porous rock or tumor vasculature---can be described by examining local interactions of two adjacent channels (pores) using a pore doublet model. However, the traditional pore doublet model does not take into account the region at the interface of the two fluids, and thus the applicability of this model for low aspect ratio pores is unknown. Here we show using computational fluid dynamics (CFD) that traditional pore doublet models begin to break down for lower channel aspect ratios due to increased energy dissipation in the fluid interface region. We also show that our pore doublet model is able to extend previous models, elucidating network level behavior from a local response. Second, we focus on the generation of highly uniform droplets. When air is blown in a straw near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate oscillatory regime that can create monodisperse aerosols from periodic angled jets. The underlying mechanisms responsible for this highly periodic regime are not well understood. We present experimentally validated scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. This mechanism has the potential to aerosolize microorganisms in the bulk fluid. Third, we look at the role of fluid stresses on nearby biological life. In the biotechnology industry mammalian cells are grown in aerated tanks where locally elevated stresses---created by bubbles rapidly changing shape---can be high enough to kill nearby cells; however the effect of elevated stresses on cells at the timescales of these bubble events is unclear. Here we investigate the effect on cell viability from fluid stresses created by a bubble undergoing topological change, using a combination of CFD, numerical particle tracking, and experimental microfluidics. Using this approach we elicit an overall bubble-induced effect on a cell population's viability. Finally, we examine the role stresses can have on bacterial aerosolization. A key component of the airborne infection pathway is the survival of the pathogen during aerosolization pinch-off processes. Due to a rapidly rearranging interface, pinch-off processes have the potential to generate an enormous amount of hydrodynamic stress in the surrounding fluid. However, the magnitude and duration of the hydrodynamic stresses in these droplets is unknown. We show using numerical simulations the spatial and temporal hydrodynamic stress history of microscale aerosol droplets produced by the central jet of collapsing bubbles. This stress history can then be linked to the stress tolerance of various bacteria allowing for the creation of a new stress-based metric for bacterial survival during aerosolization.