Deformability while remaining viable is an important mechanical property of cells. Red blood cells (RBCs) deform considerably while flowing through small capillaries. The RBC membrane can withstand a finite strain, beyond which it ruptures. The classical yield areal strain of 2-4% for RBCs is generally accepted for a quasi-static strain. It has been noted previously that this threshold strain may be much larger with shorter exposure duration. Here we employ an impulse-like forcing to quantify this yield strain of RBC membranes. In the experiments, RBCs are stretched within tens of microseconds by a strong shear flow generated from a laser-induced cavitation bubble. The deformation of the cells in the strongly confined geometry is captured with a high-speed camera and viability is successively monitored with fluorescence microscopy. We find that the probability of cell survival is strongly dependent on the maximum strain. Above a critical areal strain of ∼40%, permanent membrane damage is observed for 50% of the cells. Interestingly, many of the cells do not rupture immediately and exhibit ghosting, but slowly obtain a round shape before they burst. This observation is explained with structural membrane damage leading to subnanometer-sized pores. The cells finally lyse from the colloidal osmotic pressure imbalance.
One of the earliest events in cellular mechanotransduction is often an increase in intracellular calcium concentration associated with intracellular calcium waves (ICWs) in various physiologic or pathophysiologic processes. Although cavitation-induced calcium responses are believed to be important for modulating downstream bioeffects such as cell injury and mechanotransduction in ultrasound therapy, the fundamental mechanisms of these responses have not been elucidated. In this study, we investigated mechanistically the ICWs elicited in single HeLa cells by the tandem bubble-induced jetting flow in a microfluidic system. We identified two distinct (fast and slow) types of ICWs at varying degrees of flow shear stress-induced membrane deformation, as determined by different bubble standoff distances. We showed that ICWs were initiated by an extracellular calcium influx across the cell membrane nearest to the jetting flow, either primarily through poration sites for fast ICWs or opening of mechanosensitive ion channels for slow ICWs, which then propagated in the cytosol via a reaction-diffusion process from the endoplasmic reticulum. The speed of ICW ( ) was found to correlate strongly with the severity of cell injury, with in the range of 33 μm/s to 93 μm/s for fast ICWs and 1.4 μm/s to 12 μm/s for slow ICWs. Finally, we demonstrated that micrometer-sized beads attached to the cell membrane integrin could trigger ICWs under mild cavitation conditions without collateral injury. The relation between the characteristics of ICW and cell injury, and potential strategies to mitigate cavitation-induced injury while evoking an intracellular calcium response, may be particularly useful for exploiting ultrasound-stimulated mechanotransduction applications in the future.
We report about an intriguing boiling regime occurring for small heaters embedded on the boundary in subcooled water. The microheater is realized by focusing a continuous wave laser beam to about 10 µm in diameter onto a 165 nm-thick layer of gold, which is submerged in water. After an initial vaporous explosion a single bubble oscillates continuously and repeatably at several 100 kHz. The microbubble's oscillations are accompanied with bubble pinch-off leading to a stream of gaseous bubbles into the subcooled water. The self-driven bubble oscillation is explained with a thermally kicked oscillator caused by the non-spherical collapses and by surface pinning. Additionally, Marangoni stresses induce a recirculating streaming flow which transports cold liquid towards the microheater reducing diffusion of heat along the substrate and therefore stabilizing the phenomenon to many million cycles. We speculate that this oscillate boiling regime may allow to overcome the heat transfer thresholds observed during the nucleate boiling crisis and offers a new pathway for heat transfer under microgravity conditions.Introduction.-The thermal energy transfer from a heater into a liquid is greatly increased once the boiling temperature is reached and vapor bubbles are formed. Then the previous convective driven flow becomes advected by the growing and detaching bubbles rising into the bulk under the action of gravity. By increasing the temperature of the heater further more bubbles are nucleated until a continuous vapor layer is formed [1]. In this so-called film boiling regime heat transfer is strongly reduced; this regime limits the design and efficiency of common heaters. To overcome this boiling crisis, research is focused on the enhancement of heat transfer while avoiding the transition to film boiling. Current promising approaches are the texturing of the heater surface [2] or the use of thin electrically heated wires [3]. Here we report a nucleate boiling regime on a flat substrate where the vapor bubble does not detach from the surface yet transports heat through a flow caused by bubble oscillations and thermocapillary stresses. This letter starts with a description of this unexpected oscillate boiling regime and relates the phenomenon with reports from literature having some similarity before we provide two simple models. The first model, based on a set of ordinary differential equations (ODEs), explains the orgin of the bubble oscillation and the dependency of the oscillation frequency on the heater power. The second model describes the thermal gradients and the resulting thermocapillary flow using a Navier Stokes solver which is compared to the experimental measurements. This novel oscillate boiling regime may allow to overcome the film boiling regime through an auto-oscillatory flow from a constant heat input.
Chemistry. It incorporates referee's comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document.
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