Abstract
Evaporation is a key phenomenon in the natural environment and in many technological systems involving capillary structures. Understanding the evaporation front dynamics enables the evaporation rate from microfluidic devices and porous media to be finely controlled. Of particular interest is the ability to control the position of the front through suitable design of the capillary structure. Here, we show how to design model capillary structures in microfluidic devices so as to control the drying kinetics. This is achieved by acting on the spatial organization of the constrictions that influence the invasion of the structure by the gas phase. Two types of control are demonstrated. The first is intended to control the sequence of primary invasions through the pore space, while the second aims to control the secondary liquid structures: films, bridges, etc., that can form in the region of pore space invaded by the gas phase. It is shown how the latter can be obtained from phyllotaxy-inspired geometry. Our study thus opens up a route toward the control of the evaporation kinetics by means of tailored capillary structures.
Highlights
Evaporation from capillary structures is encountered in many natural or technological systems and processes such as the evaporation from soils[1] and from fuel cells[2], the two-phase heat transport cooling devices used in the electronics cooling industry[3], evaporation-driven engines[4], the formation of porous coatings[5], the drying of porous media[6], and evaporation with the porous wicks used in several consumer products, such as air-fresheners and insect repellents, for dispensing volatile compounds into the air
The development of advanced technologies, such as 3D printing and other additive manufacturing techniques offers new possibilities for designing porous structures with properties tailored to specific applications. This reinforces the need for a better understanding of the impact of microstructure on the evaporation process so as to improve the effectiveness of capillary structures used in the applications
There are two possible ways of controlling δ(t): firstly, through the control of the main liquid – gas invasion front within the capillary structure and, secondly, through the control of capillary liquid films along the solid walls in the regions invaded by the gas phase
Summary
The very first period (VFP) is similar in the three systems and corresponds to evaporation until the invasion front is pinned on the outermost row of cylinders. A very short last period (not shown in Fig. 3c) corresponds to the evaporation of the residual liquid still present along a fraction of each spiral after the invasion front has reached the centre of the device. The phyllotaxy-inspired device leads to a quasi-constant evaporation rate period (CRP) since the liquid bridge chains remain attached at the tips of spirals for a long time. The longer the chains of liquid bridges stay attached to the outermost pillar in each spiral, the longer is the period of high evaporation rate
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