Marangoni-driven instabilities of an evaporating liquid-vapor interface

The Impact of Marangoni and Buoyancy Convections on Flow and Segregation Patterns during the Solidification of Fe-0.82wt%C Steel

Due to the high computational costs of the Eulerian multiphase model, which solves the conservation equations for each considered phase, a two-phase mixture model is proposed to reduce these costs in the current study. Only one single equation for each the momentum and enthalpy equations has to be solved for the mixture phase. The Navier-Stokes and energy equations were solved using the 3D finite volume method. The model was used to simulate the liquid-solid phase transformation of a Fe-0.82wt%C steel alloy under the effect of both thermocapillary and buoyancy convections. The alloy was cooled in a rectangular ingot (100 × 100 × 10 mm3) from the bottom cold surface to the top hot free surface by applying a heat transfer coefficient of h = 600 W/m2/K, which allows for heat exchange with the outer medium. The purpose of this work is to study the effect of the surface tension on the flow and segregation patterns. The results before solidification show that Marangoni flow was formed at the free surface of the molten alloy, extending into the liquid depth and creating polygonized hexagonal patterns. The size and the number of these hexagons were found to be dependent on the Marangoni number, where the number of convective cells increases with the increase in the Marangoni number. During solidification, the solid front grew in a concave morphology, as the centers of the cells were hotter; a macro-segregation pattern with hexagonal cells was formed, which was analogous to the hexagonal flow cells generated by the Marangoni effect. After full solidification, the segregation was found to be in perfect hexagonal shapes with a strong compositional variation at the free surface. This study illuminates the crucial role of surface-tension-driven Marangoni flow in producing hexagonal patterns before and during the solidification process and provides valuable insights into the complex interplay between the Marangoni flow, buoyancy convection, and solidification phenomena.

Marangoni-induced reversal of meniscus-climbing microdroplets

Small water droplets or particles located at an oil meniscus typically climb the meniscus due to unbalanced capillary forces. Here, we introduce a size-dependent reversal of this meniscus-climbing behavior, where upon cooling of the underlying substrate, droplets of different sizes concurrently ascend and descend the meniscus. We show that microscopic Marangoni convection cells within the oil meniscus are responsible for this phenomenon. While dynamics of relatively larger water microdroplets are still dominated by unbalanced capillary forces and hence ascend the meniscus, smaller droplets are carried by the surface flow and consequently descend the meniscus. We further demonstrate that the magnitude and direction of the convection cells depend on the meniscus geometry and the substrate temperature and introduce a modified Marangoni number that well predicts their strength. Our findings provide a new approach to manipulating droplets on a liquid meniscus that could have applications in material self-assembly, biological sensing and testing, or phase change heat transfer.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Formation, Evolution, and Extinction of Standing Waves in Evaporation from Pores

We report on the formation, evolution, and extinction of standing waves (SWs) detected by infrared measurements at the upper region of a curved meniscus interface pinned at the mouth of a horizontally positioned capillary pore. The SWs are clear and strong in acetone but absent in ethanol for both tube sizes investigated (1-2 mm diameter). Dependent upon the tube size and the initial liquid filling ratio, the SWs start sooner for a lower filling ratio. The intriguing experimental observation is that the SWs disappear at a specified liquid length between the receding meniscus and the one pinned at the tube mouth, which seems to depend strongly upon the tube size and independent of the initial liquid filling ratio. The origin of the SWs could be due to the strong interaction between surface tension and gravity, which also generates oscillatory periodic Marangoni flow in the meniscus liquid phase.

Thermocapillarity in Microfluidics-A Review

This paper reviews the past and recent studies on thermocapillarity in relation to microfluidics. The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed. The thermocapillary-driven mass transfer (the so-called Benard-Marangoni effect) can be observed in liquid films, reservoirs, bubbles and droplets that are subject to the temperature gradient. Since the contribution of a surface tension-driven flow becomes more prominent when the scale becomes smaller as compared to a pressure-driven flow, microfluidic applications based on thermocapillary effect are gaining attentions recently. The effect of thermocapillarity on the flow pattern inside liquid films is the initial focus of this review. Analysis of the relation between evaporation and thermocapillary instability approves the effect of Marangoni flow on flow field inside the drop and its evaporation rate. The effect of thermocapillary on producing Marangoni flow inside drops and liquid films, leads to actuation of drops and bubbles due to the drag at the interface, mass conservation, and also gravity and buoyancy in vertical motion. This motion can happen inside microchannels with a closed multiphase medium, on the solid substrate as in solid/liquid interaction, or on top of a carrier liquid film in open microfluidic systems. Various thermocapillary-based microfluidic devices have been proposed and developed for different purposes such as actuation, sensing, trapping, sorting, mixing, chemical reaction, and biological assays throughout the years. A list of the thermocapillary based microfluidic devices along with their characteristics, configurations, limitations, and improvements are presented in this review.

Three-dimensional Marangoni cell in self-induced evaporating cooling unveiled by digital holographic microscopy

A digital holographic microscope has been used to trace the trajectory of a tracer particle inside the liquid phase of an evaporating meniscus formed at the mouth of a 1-mm2 borosilicate tube filled with ethanol. The Marangoni flow cells are generated by the self-induced differential evaporating cooling along the meniscus interface that creates gradients of surface tension which drive the convection. The competition between surface tension and gravity forces along the curved meniscus interface disrupts the symmetry due to surface tension alone. This distorts the shape of the toroidal Marangoni vortex. Thermocapillary instabilities of the evaporating meniscus are reported by analyzing the trajectories of the tracer particle. It is found that the trajectory of the tracer particle makes different three-dimensional loops and every four loops it returns to the first loop. By analyzing several loops it was found that the characteristic frequency of the periodic oscillatory motion is around 0.125 Hz.

Use of IR thermography to investigate heated droplet evaporation and contact line dynamics

In this paper we present the results of an experimental study investigating interfacial properties during the evaporation of sessile water droplets on a heated substrate. This study uses infrared thermography to map the droplet interfacial temperature. The measurements evidence nonuniform temperature and gradients that evolve in time during the evaporation process. A general scaling law for the interfacial temperature is deduced from the experimental observations. A theoretical analysis is performed to predict the local evaporation rates and their evolution in time. The use of energy conservation laws enabled us to deduce a general expression for the interfacial temperature. The comparison between the theory and experiments shows good agreement and allows us to rationalize the experimental observations. The thermography analysis also enabled the detection of the three-phase contact line location and its dynamics. To our knowledge, such measurements are performed for the first time using thermography.

Order-of-magnitude increase in flow velocity driven by mass conservation during the evaporation of sessile drops

We report on a dramatic order-of-magnitude increase in flow velocity within pinned evaporating droplets toward the end of their lifetime. The measurements were performed using high-speed microparticle image velocimetry. The study revealed interesting observations about the spatial and temporal evolution of the velocity field. The profile along the radius of the droplet is found to exhibit a maximum toward the three phase contact line with flow oscillations in time in this region. Additional optical measurements allowed further analysis of the observed trends. Analysis of the potential mechanisms responsible for the flow within the droplet demonstrated that these observations can be satisfactorily explained and accounted for by mass conservation within the droplet to compensate for evaporation.

Recent advances on thermocapillary flows and interfacial conditions during the evaporation of liquids

Thermocapillary convection has a very different history for water than for other liquids. For water, several studies have pointed to the lack of evidence supporting the existence of thermocapillary (or Marangoni) convection. Other studies have given clear evidence of its existence and of the role it plays during steady-state water evaporation. We examine both sets of data and suggest a reason for the difference in the interpretation of the experimental data. For organic liquids, the evidence of thermocapillary convection has been clearly documented, but the issues are the type of flow that it generates during steady-state evaporation. We review the measurements and show that the flow field of the evaporating liquid is strongly affected by the presence of the thermocapillary convection. When the results obtained from both water and organic liquids are compared, they give further insight into the nature of thermocapillary convection.