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Three-dimensional numerical simulations were performed to study the liquid flow inside split T-type micromixers, investigating the effects of various operating and design parameters on the mixing process. While for small Reynolds numbers, the mixing efficiency of a split T-micromixer is larger than that of a simple T-micromixer, it was found that, when Re = 100, mixing is further enhanced by the formation of vortex pairs. This is not only a function of the flow rate, but it depends also on the geometry of the device and on the ratio between the flow rates in the inlet sub-channels. In general, at a given flow rate, vertical splits induce a much better mixing than horizontal splits, while the reverse may occur for strongly unequal inlet flow rates.

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In this work, the Marangoni convection in the liquid phase of an evaporating meniscus interface in open air has been studied for varying contact angles. Ethanol undergoes self-evaporation inside a capillary tube of borosilicate glass with internal diameter of 1 mm. The evaporation is not uniform along the meniscus interface pinned at the capillary tube mouth, and this creates a gradient of temperature between the wedge and the centre of the meniscus. It is this temperature difference and the scale (1 mm) that generate a gradient of surface tension that is acknowledged to drive the vigorous Marangoni convection in the meniscus liquid phase. In previous studies of this configuration, the meniscus has mainly been concave and for this reason, other researchers attributed the differential temperature along the meniscus to the fact that the meniscus wedge is closer to the tube mouth and also further away from the warmer liquid bulk than the meniscus centre. The present study investigates concave, flat and convex meniscus by using a syringe pump that forces the meniscus to the wanted shape. With the present investigation, we want to further demonstrate that it is instead the larger evaporation at the meniscus triple line near the wedge that controls the phenomenon. Flow visualization and infrared temperature measurements have been performed. For concave and convex meniscus, the temperature measurements are in line with the predicted trend; the Marangoni vortices for these two menisci shapes spin in the same direction according to the temperature differences along the meniscus. For a flat meniscus, an intriguing experimental evidence has been found: the temperature difference is inverted with respect to concave and convex menisci, but surprisingly, the Marangoni vortices spin in the same direction than for concave and convex menisci.