Session H26: Surface Tension Effects: Interfacial Phenomena I

Suppressing Marangoni instability using parametric forcing

When a fluid layer with a free surface is subjected to a vertical parametric forcing, the free surface deforms on account of the Faraday instability. In the absence of the parametric forcing and when the fluid layer is heated from below, surface tension gradients lead to Marangoni instability beyond a critical temperature gradient, characterized by a Marangoni number. In this work, it is shown that these flows can be suppressed through periodic mechanical forcing. Analysis shows that if the Marangoni number is above the critical value, periodic forcing with a finite amplitude is required to quench the Marangoni flows. Suppressing these flows is of importance in applications in liquid encapsulated crystal growth for the manufacture of semiconductors and in additive manufacturing in microgravity environments.

    

Modulation instability of Marangoni roll convection in a layer with deformable interface covered by insoluble surfactant

Stationary one-dimensional roll patterns are typical in several instability problems. Here we investigate the stability of stationary Marangoni rolls in a liquid layer covered by insoluble surfactant with respect to the longwave modulation. The nonlinear interaction between three longwave modes: surface deformation, convection mode described by roll’s amplitude and surfactant concentration disturbance is considered near the instability threshold, Ma-Ma_c=O(δ²) (δ is a small parameter of supercriticality),  by means of modified Newell-Whitehead amplitude equations.  With respect to transverse modulation, the problem has two different distinguished limits. The first one, with transverse modulation wavenumber proportional to δ^1/2, is prescribed by the linear stability theory and the second one, with wavenumber proportional to δ, comes from the long-wave surface deformation and surfactant distribution. Both cases are studied separately. The stability maps are plotted on the parameter plane (β,G), where β is the Biot number and G is the Galileo number, for different values of surfactant concentration.   

Achieving Stable Patterns in Multicomponent Polymer Thin Films Using Marangoni and van der Waals Forces

Topographically patterned polymer films are promising for applications such as fabrication of integrated circuit elements in microelectronics, optical gratings, photonic crystals, and sensors and actuators. Liquid-air interfaces can be deformed by surface-tension gradients to create topography, a phenomenon useful for polymer film patterning. A recently developed method creates these gradients by photochemically patterning a solid polymer film. Heating the film to the liquid state leads to flow driven by the patterned surface-tension gradients, but capillary leveling and diffusion of surface-active species facilitate eventual dissipation of the topography. However,  experiments demonstrate that using blends of high- and low-molar-mass polymers can considerably delay the decay in topography. To gain insight into this observation, we develop a model based on lubrication theory that yields coupled nonlinear partial differential equations describing how the film height and species concentrations evolve with time and space. Incorporation of a non-monotonic disjoining pressure is found to significantly increase the lifetime of topographical features, making the model predictions qualitatively consistent with experiments. A parametric study reveals the key variables controlling the kinetics of film deformation and provides guidelines for photochemically induced Marangoni patterning of polymer films.   

Destabilizing Marangoni flow in an evaporating binary liquid lens

Evaporating multi-component droplets are ubiquitous in nature. They are very complex systems, governed by many parameters, such as volatility and surface tension, to name but two.

One example of an unstable multi-component evaporating system is Marangoni Bursting as first studied by Keiser et al. [PRL 118, 074504 (2017)].

Surface tension and evaporation induce an alcohol/water droplet floating on an oil bath to burst into daughter droplets. The different volatilities and surface tensions of the two components generate a Marangoni flow, which leads to an instability at the droplet rim.

Here we present novel experimental observations of the flow in the mother droplet using high-speed microscopic imaging to characterize contact line motion, rim instability, droplet pinch-off, and the internal flow field. The Marangoni-driven velocity increases with an increasing alcohol concentration φ_0 in the droplet, confirming previous studies. However, towards the unstable contact line the flow slows down and, independent of φ_0, reaches an unexpected plateau value of constant velocity.

We compare our experimental results with numerical simulations, which offer us – previously inaccessible – insights into the phenomenon, such as the interface shape between binary mother droplet and oil bath.   

Spontaneous spinning of a dichloromethane drop upon a CTAB solution

An intriguing phenomenon arises when a dichloromethane drop is laid upon a 10 mM solution of CTAB, a surfactant. The circular drop contour starts to oscillate, leading potentially to the triangular mode 3 of deformation. The system keeps evolving and turns into a self-rotating system with a characteristic helix-like shape. The spinning lasts until the dichloromethane drop entirely dissolves and evaporates, which takes approximately 12 seconds for a 20 uL initial drop. To study the phenomenon, and elucidate the origin of this spontaneous symmetry breaking, we record the drop spinning with a diascopic optical setup. This device allows us to determine the drop shape and measure the spinning velocity. Surprisingly, this velocity is found to be very stable during the spinning phase and does not depend on the depth of water. In a complementary approach, we determine the horizontal fluid velocity field around the drop at several distances from the surface by using two-dimensional PIV. Owing to the three surface tensions at the contact line, a radial Marangoni flow moves the water surface outward. This primary diverging flow combines with flows resulting from the high density (1.33) and volatility of the dichloromethane, resulting in a complex and composite pattern around the drop.   

Transient velocity and temperature distributions in a water drop evaporating on a non-isothermal substrate

The particle image velocimetry (PIV) technique and an infrared camera were used to measure a water drop’s transient velocity and the surface temperature distributions undergoing evaporation. The drop’s bulk temperature was measured using a thermistor. The drop was placed on a substrate, and its initial temperature was varied compared to that of the substrate, which changed the thermo-capillary force acting on the drop. The drop’s bulk and surface temperatures initially varied rapidly with time and then reached a steady value. The time a drop takes to attain the steady value depends on the substrate’s conductivity and, as expected, increased with increasing drop volume.   

Non-uniform absorption and evaporation at a droplet interface

A water droplet on a solid substrate can evaporate and simultaneously absorb vapors from the surrounding atmosphere. If alcohol vapors are near the water droplet, the vapour-driven solutal Marangoni flows are generated, which can be used for various applications including droplet splitting, mixing, and coating. To control the solutal Marangoni flow, it is important to understand the molecule transport at the vapor-liquid interface. We believe that the vapour absorption and evaporation should be considered at the same time. However, it has been rarely investigated for these features. Thus, in this work, we experimentally and numerically investigated how the solutal Marangoni flow patterns changed depending on vapor distributions and different absorption conditions. Both experimental and numerical results showed that the absorption and evaporation rates are non-uniform along the radial direction and the two phenomena competed each other, so that the internal flow structures were changed depending on the contact angle of the droplet and the alcohol vapor source location. During the talk, physical arguments will be provided to explain the working mechanism of the solutal Marangoni flow.   

Not Participating

We study the propulsion at the surface of water of small objects (2cm) called the Marangoni boats. Several studies have been conducted on similar swimmers the camphor boats, a non soluble surfactant. However, it has been shown that one can generate a controlled Marangoni flow thanks to soluble surfactants such as TTAB. The solubility adds a complexity to the Marangoni flow, hence in this study we try to characterise the displacement of the swimmers by probing the effects of the solubility and the concentration of the surfactant solution. The boats are composed of two parts. The first is the floater cut out of a transparent plastic sheet. And the motor is a filter paper soaked with a souble surfactant solution sticked to the floater with nail polish. When the boat is deposited at the surface of a water layer, the surfactant spreads at the surface decreasing the surface tension at the back of the boat. It creates a gradient of surface tension between the bow and the stern of the boat, thus a difference in capillary force appears resulting in the propulsion of the boat forward. We record and track the motion of the boat with a camera and the trackpy python algorithm. From the trajectory we can measure the velocity of the boat while it swims. We perform the experiments with four different surfactants (HTAC, TTAB, DoTAB, DeTAB) with different Critical micellar concentrations  (respectively 1.6 mM, 4mM, 15 mM and 65mM). In addition we propose a simple model to predict the initial velocity based on the balance between the capillary force as the driving force and the skin friction force opposed to the movement of the boat. The agreement between the experimental data and the model seems good. We observed that the initial velocity of the boat and the evolution of the velocity versus the time changes with the concentration but also the solubility of the surfactant. The more soluble the surfactant is the less efficient it is to propel the boat .   

On the spreading of liquid lenses over rheologically complex liquid films

Droplet spreading over non-Newtonian liquid substrates occurs in a variety of technological and biological applications. A characteristic example of the latter is the delivery of aerosol drugs with droplet spreading taking place along the airway mucus of the lungs.. Here, we investigate the spreading dynamics of a liquid lens over a thin fluid layer which exhibits rheolgically complex behaviour. We consider liquids films following the Ostwald-de Waele constitutive equation or the Oldroyd-B model in the limit of weak viscoelasticity. In the limit of both a thin droplet and a thin subphase, we employ lubrication theory to derive a coupled system of evolution equations for the interface positions and the resulting governing equations are solved numerically using the finite element method. The results of an extensive parametric analysis to examine the effects of the physical parameters and rheological characteristics on the flow will be discussed.   

Evaporating polygonal shape of multi-component droplets in a confined space suppresses coffee-ring effects

To control and suppress coffee-ring effects are crucial to achieve uniform patterns from a droplet evaporation. To date, a non-uniform evaporative flux along the radial direction is mainly studied, which is the case of a constant radius of curvature of the droplet. It is reported that if the contact area shape has a non-uniform curvature along the azimuthal direction, the coffee-ring flow accelerates toward the vertex. Then, the coffee-ring patterns become much more serious at the vertex. In this study, we showed that if a multi-component droplet evaporated inside a confined space, the coffee-ring patterns completely disappeared. Not only a circular droplet but also non-spherical asymmetric droplets, e.g., triangle, square, and hexagon, were investigated. From PIV experiments, we observed that the confinement geometry induces a radially-inward circulating solutal Marangoni flow by capturing the evaporated vapors regardless of the droplet shape. Finally, we obtained the coffee-ring-less dried patterns after a complete evaporation in the confined space. During the talk, we will provide scaling arguments and theoretical model to explain the main mechanism. We expect that it can be widely utilized in a QLED color filter with various shapes of the patterns in a display field.