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The detailed structure of a test microdevice comprised of a microchannel array with microfluidic transistors. (a) The top-view of a microfludic test chip. (b) An exploded view of auxilliary channels and main channels, connected through cross-junctions highlighted by the blue dashed line. (c) A scanning electron microscope (SEM) image of a microfluidic transistor. (d) The schematic mechanism of a microfluidic transistor.
Visualization of a bubble growing/collapsing cycle in an auxiliary channel at a volumetric flow rate of 11.2 μL/s and an effective heat flux of 111.7 W cm−2 (for one micro fluidic transistor). White and dark areas indicate vapor and liquid phase. (a) Single phase liquid flows in the auxiliary channel. (b) A bubble nucleates on the wall. (c) The bubble explosively grows and is confined. (d) Liquid thin film between a confined bubble and the solid wall is drying out. (e) A confined bubble rapidly collapses when its front cap directly contacts with subcooled liquid and results in a bubble shrinkage and collapse. (f) A fluid flow from the auxiliary channel is jetted into the main channel by the pressure gradient established from the rapid bubble collapse (enhanced online). [URL: http://dx.doi.org/10.1063/1.4745782.2]10.1063/1.4745782.2
Visualization of a detailed bubble growth/collapse cycle modulated by a microfluidic transistor at a mass flux of 150 kgm−2 s−1 (volumetric flow rate at 7.5 μL/s) and heat flux of 184 W cm−2. Arrows show the flow directions. The whole cycle was self-sustained under a constant heat and mass flux (enhanced online). [URL: http://dx.doi.org/10.1063/1.4745782.1]10.1063/1.4745782.1
HF-TPO model and experimental measurements. (a) A lumped system model. and are transient flow resistances in auxiliary channels as illustrated in (b) and (c) (the unit of R reads as “Pa s m−3”). and denote flow resistance from a restrictor and the cross junction (i.e., from “source” to “drain”), respectively. (b) Periodic flow resistance in auxiliary channels during HF-TPO cycles within a period . (c) Periodic flow resistances during a BGC cycle in auxiliary channels were predicted by physical models and numerically fitted by a logistic function, where , , and denote the bubble growing time prior to attaching on walls, the duration for liquid film evaporating, and the time of a bubble sustaining and collapsing, respectively. (d) Pressure drop oscillations around mean value in the main channel were measured by transducers at 1 kHz sampling rate and predicted numerically at the heat flux of 125 W/cm2 and mass flux of 258 kg/m2·s. (e) TPO frequency measured by a high-speed camera ranges from 134 Hz to 613 Hz under various heat and mass fluxes and were compared with data on flow oscillations from other researchers (see Refs. 9 and 10).
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