Marangoni convection in evaporating meniscus with changing contact angle

Abstract

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.

Transport of Colloids along Corners: Visualization of Evaporation-Induced Flows beyond the Axisymmetric Condition

“…In the Supporting Information, we have included a video showing the motion of the particles in the EtOH–glass configuration. These secondary flows may well be formed by transverse temperature gradients, leading to Marangoni vortical structures, as seen in axisymmetric geometries. , However, if we look at the values of the factor γ T /ηα in Table , linked to the Marangoni number, the highest value is for water, which, however, shows rectilinear particle paths. Hence, the transverse motion is not uniquely determined by lateral temperature gradients, but also by lateral mass flow, which can occur precisely in fluids having very low surface energies or high evaporation rates.…”

Modified coffee rings for quasi‐1D interconnects

“…The radial velocity of the drop is inversely proportional to the height of the drop. Evaporation causes a temperature drop at the liquid–vapor interface , and leads to the creation of a temperature gradient as a function of depth . The interface contracts toward the center of the coffee ring, causing a Marangoni‐type flow .…”

Ultrahigh evaporative heat transfer measured locally in submicron water films

“…Experimental measurements have probed evaporation in the macroscopic extended meniscus, where the evaporation heat transfer coefficient takes on its bulk value of 0.001-0.1 MW/m 2 -K 5-7 . Theory intriguingly suggests an up to three orders-of-magnitude enhancement of the evaporation rate, and hence the heat transfer rate, in the thin film region of the meniscus, but these predictions have not yet been validated [8][9][10][11][12][13] .The evaporation rate from a thin liquid film is controlled by a competition between the film thermal resistance and a suppressed liquid pressure. The latter results from the disjoining pressure P d , which measures the interaction strength between the solid substrate and the liquid film.…”

“…The efficiency of solar thermal generation [22][23][24] and desalination processes 25,26 will also be improved by engineering evaporation in thin liquid films to obtain high mass fluxes.Experimental thin liquid film evaporation studies are often conducted by extracting the temperature profile along a meniscus on a heated surface. Infrared cameras [11][12][13] and thermocouples [8][9][10]27 with spatial resolution from 6 μm to 2 mm have been used to measure local temperatures. Reported heat flux and/or temperature profiles demonstrate enhanced heat transfer near the edge of the meniscus (i.e., the three-phase contact line).…”

“…Experimental measurements have probed evaporation in the macroscopic extended meniscus, where the evaporation heat transfer coefficient takes on its bulk value of 0.001-0.1 MW/m 2 -K 5-7 . Theory intriguingly suggests an up to three orders-of-magnitude enhancement of the evaporation rate, and hence the heat transfer rate, in the thin film region of the meniscus, but these predictions have not yet been validated [8][9][10][11][12][13] .…”