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Liquid evaporation induced self-driven microvortex and its applications
The control of flow field morphology has been an important fundamental problem in the field of microfluidics. Among them, the construction and application of vortices have attracted the most attention of researchers. An effective way to construct vortices in microfluidic channels is to introduce some obstacles in them or to design flow channels with special geometrical structures. Vortices are formed when the Reynolds number (Re) of the flow reaches high values (Re>20). However, these vortices do not form in the region of low Reynolds number flow (Re<1, called Stokes flow or creeping flow). However, achieving high Reynolds numbers in microchannels requires very high flow rates. For some important applications, such as the generation of localized dissolution, fluid mixtures and chemical reactions, a relatively low velocity is required, and most of the flows in microfluidic applications belong to peristaltic flow. Therefore, the establishment of vortex fields in peristaltic flows remains challenging and of great importance.
Recently, a domestic research team proposed a method to construct micrometer-scale vortices under creeping flow using the Marangoni effect. The Marangoni effect is induced by the surface tension gradient at the water-air interface, which is capable of generating a force on the fluid surface. In the presence of such surface forces, vortex flow can be generated in the region of creeping flow. This work provides an evaporation-induced self-driven vortex system by microstructural manipulation of the Marangoni flow. Wherein the Marangoni flow is formed by inhomogeneous evaporation of the surfactant solution. The inhomogeneous evaporation creates an inhomogeneous distribution of surfactant concentration on the liquid surface, which results in the formation of a Marangoni flow. In the experiment, the surfactant solution was sandwiched between a microstructured silicon substrate and a glass lid. As a result, the fluid was confined to form a thin layer in a quasi-two-dimensional space and the fluid interface became narrow, in which case the Marangoni flow exhibited a shallow surface flow behavior. As the surfactant concentration increases, the convective distance of the Marangoni flow increases. On the basis of long convection distance, the surface flow can be transformed into vortex flow by microstructurally folding the fluid interface to form some specific shapes. The morphology of the vortex depends on the shape of the fluid interface and the microstructure in the fluid, and the vortex pattern in the structure can be accurately predicted using numerical simulation. Using this vortex flow, on the one hand, it is possible to rotate micro-objects at the micrometer scale, and on the other hand, it realizes the transport and enrichment of micrometer particles with particle size screening, and is able to enrich particles in vortex flow. This vortex construction method is expected to be widely used in the field of microfluidics.
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