Experimental and numerical investigation of the equilibrium geometry of liquid lenses

Spreading and Capillary Imbibition of Viscous Oil Lens into an Open-Cell Porous Structure

Oil pollution in the ocean is becoming more and more of a serious issue, which increases interest in both ways for combating its cause and methods for observing and monitoring how oil spreads. A promising approach based on an optical method with empirical relations for selected viscous oil-water systems is presented. Based on a modified melamine sponge (MMS), the microscopic spreading and oil capillary penetration phenomenon of the porous structure were investigated. The objective of this study is 2-fold: (i) to present a more thorough experimental description of the spreading of viscous oil lens on the water surface and capillary action of oil lens into MMS porous structure; and (ii) to provide a theoretical description that helps to explain some of the observed behavior. With knowledge of δ ∞ 2 = - 2 S ρ W / g ρ O ( ρ W - ρ O ) , we can determine the spreading coefficient S. It needs to be pointed out that the oil lens floating on the water surface does satisfy Neumann's rule as the spreading coefficient of the air-oil-water system is negative (- 9.8 mN/m), indicating the ability to form a stable oil lens with thickness δO = 3.04 mm and radius RL = 38.64 mm after 60 min of spreading test. Furthermore, to better understand the capillary phenomena from a mechanical approach, an oil lens in contact with the surface of the MMS porous structure, by in-depth visualization, is properly defined as the balance of forces acting. Finally, as an illustration of this method, we utilized this approach to obtain the equilibrium height of the capillary rise and take it into account in terms of effective material thickness.

Synthesis of silica aerogel films in liquid molds

By virtue of their low density and thermal conductivity, aerogels constitute attractive thermal insulators. Of those, aerogel films are best suited for thermal insulation in microsystems. Processes for the synthesis of aerogel films with thicknesses smaller than 2 µm or thicker than 1 mm are well established. However, for microsystems films in the range of a few microns and up to several hundred microns would be beneficial. To circumvent the present limitations, we describe a liquid mold made of two immiscible liquids, used here to produce aerogel films thicker than 2 µm in a single molding step. Following gelation and aging, the gels were removed from the liquids and dried using supercritical carbon dioxide. In contrast to spin/dip coating, liquid molding avoids solvent evaporation from the gel's outer surface during gelation and aging, films are free-standing and have smooth surfaces. The choice of liquids determines the aerogel film thickness. As a proof of concept, 130 µm thick homogeneous and high porosity (>90%) silica aerogel films were synthesized in a liquid mold with fluorine oil and octanol. The resemblance of the liquid mold approach to the float glass technique offers the prospect of mass production of large sheets of aerogel films.

Morphology Evolution of a Volatile Liquid Lens on Another Immiscible Liquid Surface Induced by Evaporation

A theoretical model was established to predict the morphology evolution of a volatile liquid lens evaporation on another immiscible liquid substrate surface. The theoretical model considered the dynamic process of contact line motion. On the basis of the boundary conditions established at the contact line, the morphology change of the liquid lens was calculated by numerically solving the Young-Laplace differential equations for the three interfaces. The mass evaporation rate was calculated by the diffusion-controlled evaporation model. Then, an experimental system was established to record the process of a hexane lens evaporation on the surface of an ionic liquid with a depth of 4 mm. The calculated hexane lens radius variation matches well with the experimental measurements, which shows the rationality of the present model. The calculated results show that the evaporation pattern of the liquid lens follows the constant contact-angle evaporation mode for ∼70% of the lifetime. During the later stage of evaporation, the contact angle decreases, accompanied by contraction of the contact line, which is similar to the mixed evaporation mode in the later stage of sessile droplet evaporation on a solid substrate surface. Furthermore, the influences of the initial hexane lens volume and the ionic liquid temperature on the dynamic contact angle were theoretically summarized. This study helps to provide in-depth insights into regulating the lens evaporation process on another immiscible liquid substrate surface to control the particle deposition mode.

Minimum Leidenfrost Temperature on Smooth Surfaces

During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Here we demonstrate that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is ≈140 °C, corresponding to an average vapor layer thickness of 10-20  μm.

Can Liquid Lenses Increase Depth of Field in Head Mounted Video See-Through Devices?

Wearable Video See-Through (VST) devices for Augmented Reality (AR) and for obtaining a Magnified View are taking hold in the medical and surgical fields. However, these devices are not yet usable in daily clinical practice, due to focusing problems and a limited depth of field. This study investigates the use of liquid-lens optics to create an autofocus system for wearable VST visors. The autofocus system is based on a Time of Flight (TOF) distance sensor and an active autofocus control system. The integrated autofocus system in the wearable VST viewers showed good potential in terms of providing rapid focus at various distances and a magnified view.

Thermodynamic Investigation of Droplet-Droplet and Bubble-Droplet Equilibrium in an Immiscible Medium

In the absence of external fields, interfacial tensions between different phases dictate the equilibrium morphology of a multiphase system. Depending on the relative magnitudes of these interfacial tensions, a composite system made up of immiscible fluids in contact with one another can exhibit contrasting behavior: the formation of lenses in one case and complete encapsulation in another. Relatively simple concepts such as the spreading coefficient (SC) have been extensively used by many researchers to make predictions. However, these qualitative methods are limited to determining the nature of the equilibrium states and do not provide enough information to calculate the exact equilibrium geometries. Moreover, due to the assumptions made, their validity is questionable at smaller scales where pressure forces due to curvature of the interfaces become significant or in systems where a compressible gas phase is present. Here we investigate equilibrium configurations of two fluid drops suspended in another fluid, which can be seen as a simple building block of more complicated systems. We use Gibbsian composite-system thermodynamics to derive equilibrium conditions and the equation acting as the free energy (thermodynamic potential) for this system. These equations are then numerically solved for an example system consisting of a dodecane drop and an air bubble surrounded by water, and the relative stability of distinct equilibrium shapes is investigated based on free-energy comparisons. Quantitative effects of system parameters such as interfacial tensions, volumes, and the scale of the system on geometry and stability are further explored. Multiphase systems similar to the ones analyzed here have broad applications in microfluidics, atmospheric physics, soft photonics, froth flotation, oil recovery, and some biological phenomena.

A Study of the Evaporation of Hexane Lenses on an Ionic Liquid Surface: Effect of Wetting Mode

The evaporation of hexane lenses on an ionic liquid (IL) (1-butyl-3-methylimidazolium hexafluorophosphate) surface is studied. The difference between the evaporation processes of the lens on the IL surface and on a distilled water (DW) surface with the same substrate liquid depth (2.6 mm) is primarily analyzed. The variation of the lens contact diameter D_C and the deformation of the IL surface were experimentally observed. The results indicated that the spreading stage of a hexane lens was notably shorter in duration on the IL surface than on the DW surface. A hexane lens was pseudopartially wetted on the DW surface, and the plane position of the lens contact diameter remained level with the water surface throughout the evaporation process. In comparison, a hexane lens was partially wetted on the IL surface, and the plane position of the lens contact diameter was lower than the horizontal surface until the lens evaporated completely. The hexane lens evaporation on the IL surface was calculated by using the diffusion-controlled evaporation model under the constant contact angle mode. The calculated results agreed well with the experimental measurements. Finally, the evaporation of hexane lenses on the DW and the IL surfaces was compared through calculations. Although the maximum lens contact diameter on the DW surface was greater, it took a longer time for the lens to evaporate on the DW surface. This is because the more significant bending of the substrate liquid surface accelerated the lens evaporation. The results of this study offer a new approach for controlling droplet evaporation.

Interactions between polystyrene particles with diameters of several tens to hundreds of micrometers at the oil-water interface

HYPOTHESIS

The charged spherical colloidal particles at the fluid-fluid interface experience considerably strong and long-ranged electrostatic and capillary interactions. The contribution of capillary force becomes more significant as the particle size increases beyond a certain limit. The relative strengths of the two competing interactions between the spherical polystyrene particles at the oil-water interface are quantified depending on their size.

EXPERIMENTS

The studied particles, obtained using the microfluidic method, have diameters of tens to hundreds of micrometers. The scaling behaviors of the commercially available colloidal particles with diameters of ~3 μm are also compared. An optical laser tweezer apparatus is used to directly or indirectly measure the interparticle force. Subsequently, the capillary force that can be attributed to the gravity-induced interface deformation and contact line undulation is calculated and compared with the measured interaction force.

FINDINGS

Regardless of the particle diameter (~3-330 μm), the measured force is observed to decay as r^-4, where r denotes the center-to-center separation, demonstrating that the dipolar electrostatic interaction is important and that the gravity-induced capillary interaction is negligible. Furthermore, numerical calculations with respect to the undulated meniscus confirm that the magnitude of capillary interaction is significantly smaller than that of the measured electrostatic interaction.

Tuning Contact Line Dynamics and Deposition Patterns in Volatile Liquid Mixtures

The spreading of a pure, volatile liquid on a wettable substrate has been studied in extensive detail. Here we show that the addition of a miscible, nonvolatile liquid can strongly alter the contact line dynamics and the final liquid deposition pattern. We observe two distinct regimes of behavior depending on the relative strength of solutal Marangoni forces and surface wetting. Fingerlike instabilities precede the deposition of a submicron thick film for large Marangoni forces and small solute contact angles, whereas isolated pearl-like drops emerge and are deposited in quasicrystalline patterns for small Marangoni forces and large solute contact angles. This behavior can be tuned by directly varying the contact angle of the solute liquid on the solid substrate.

Life-Like Motion of Oil Drops at the Air-Liquid Interface

Generally, interactions of oil drops at the air-liquid interface mainly have two features, namely, attraction and repulsion. However, in our study, we find that the oil drops at the air-liquid interface have other interacting features, that is, the atomic-like motion and the "capture" motion. For the atomic-like motion, oil drops attract each other at a long distance, but repel when they are about to come into contact with each other. For the "capture" motion, a big oil drop can actively "capture" oil droplets like a zooplankton. In our research, we analyze interfacial forces among the oil drops. Based on the experiments and analyses, we demonstrate that the atomic-like motion of oil drops is mainly due to the lateral capillary force and the surface tension force, and the "capture" motion is mainly due to the unbalanced impact force of flow fluid around the drops. In addition, based on our results, we use the oil drops to perform many functions at the air-liquid interface. For example, the oil drops can drive an object with linear and rotational motion. When a carbon tetrachloride drop is suspended above the air-liquid interface, it can be used to control an oil droplet to pass through serpentine grooves and obstacles. In addition, the suspended carbon tetrachloride drops also can be used to rank multiple droplets with a special shape. Based on the results, our study makes it possible to use oil drops to transport materials, drive objects, and even collect droplets at the air-liquid interface.

Experimental and Theoretical Study of Evaporation of a Volatile Liquid Lens on an Immiscible Liquid Surface

The evaporation of a hexane lens on a distilled water surface was experimentally and theoretically studied. The formation of the hexane lens was recorded by a high-speed camera from the side to observe the variations of the contact diameters and contact angles. The experimental results showed that the shape variation of the hexane lens experienced the spreading stage and the evaporation stage. The spreading stage lasted for about 6% of the lens lifetime. For most time of the evaporation stage, the square of the lens contact radius decreased linearly with time, while the contact angle remained almost unchanged. During the final rapid evaporation stage (about 2% of the lens lifetime), the shape of the hexane lens changed and the lens shrank rapidly until it disappeared. A theoretical model based on diffusion-controlled evaporation under the constant contact angle mode was developed to describe the evaporation of the hexane lens on the water surface. In terms of geometry, the model assumes that a lens is composed of upper and lower spherical caps, and the apparent contact angle is defined based on the intersection of the two caps. The results calculated using the model were found to be in good agreement with the experimental data. Finally, the effects of initial lens volume, water temperature, and water surface deformation on lens evaporation were discussed through calculations. The results showed that increase in the water temperature and deformation of the water surface accelerated the evaporation process.

Vapor-Driven Transport of Different Types of Objects at the Air-Liquid Interface

Transportation and position control of objects on the surface of liquids is an important part of automation. To drive an object on the surface of a liquid, many methods have been proposed. However, these methods mainly focus on the driving of the object itself, and it is still difficult to precisely control its position. In our study, we propose a new method that uses vapor released from a suspended drop to achieve precise position control and transport of different types of objects at the air-liquid interface. These objects can be a plastic plate, a liquid marble, or an oil drop. The mechanism for controlling objects is that vapor released from a suspended drop causes a surface tension gradient around the object. When the vapor dissolves on the surface of a liquid, the surface tension of the liquid increases. Due to the surface tension gradient, the object moves from the surrounding area to the area below the suspended drop and follows the motion of the suspended drop with the trajectory of a letter. To show that the position of the objects can be precisely controlled by our method, we control the object on the center of a circle, and the maximum offset distance from the center of the circle is less than 3 mm. In addition, we also use vapor released from a suspended drop to transport an oil drop close to an object. After the drop adhered with the object, the object is driven by the oil drop. Compared with other methods that drive the motion of objects by reducing the surface tension of a liquid, our method is easy and the position of objects can be precisely controlled.

Formation, stability and hydrothermal waves in evaporating liquid lenses

We present a fascinating experimental investigation of the formation, stability and thermal patterns of evaporating liquid lenses deposited on an evaporating or non-evaporating liquid pool. The use of infra-red allowed measuring the key parameters of the lens and the pool surface temperature. We unveil the significant interaction of the lens with the underlying liquid in the pool. In particular, the contact line of the lens is deformed very significantly and we ascribe this to the combined buoyancy-thermocapillary convection cells on the surface of the liquid pool, generated by a self-induced evaporative cooling effect. We also demonstrate that the evaporative cooling is ultimately responsible for the formation of the lens, which otherwise would have not formed at ambient temperature. The depth of the pool is shown to be very influential on the stability of the volatile lens and its dynamics.

Coexistence and Sudden Entrapment between Two Dissimilar, Miscible Oil Lenses

The property of substrates is one of the important factors determining the interaction between two lenses (droplets). There likely exist different interactions between two dissimilar oil lenses (droplets) floating on the surface of a liquid phase from the interaction between two dissimilar oil droplets on a rigid substrate, for example, coalescence or coexistence. The interaction between two dissimilar oil lenses (droplets) is dependent on the intrinsic properties of both oil lenses (droplets) and external environmental factors. In this work, we investigate the contact interaction between two dissimilar, miscible oil lenses (toluene and silicone oil) on the surface of deionized water (DI water). The morphological evolution of two dissimilar, miscible oil lenses during the interaction under different experimental conditions is recorded and analyzed. The effects of the volume ratio of two dissimilar, miscible oil lenses, temperature of DI water, and viscosity of silicone oil on characteristic parameters are systematically studied. A sudden "entrapment" of a toluene lens into a silicone oil lens occurs after a period of the "mass exchange" (coexistence) between these two oil lenses. Several characteristic parameters, including the duration of the "mass exchange" and critical sizes of the toluene lens at the onset of the entrapment and after the entrapment, are found to be dependent on experimental conditions.

Self assembly of cyclic polygon shaped fluid colloidal membranes through pinning

2D fluid monolayer membranes of rod-like viruses spontaneously form in a mixture of rods and polymers through depletion attraction. The rods are uniformly oriented within the bulk and twist in a zone around the membrane edge. Surprisingly, we find that cyclic polygonal shaped colloidal membranes form when polymers are added to a mixture of long and short-thick rods with the long and short-thick rods forming the faceted core and lobes of the polygon, respectively. We demonstrate that the origin of this anisotropic shape lies in the phenomenon of spreading of one liquid over another in the presence of disorder. As a membrane of short-thick rods spreads over another of longer rods, the edge bound rods untwist to become part of the newly formed two-rod interface. However, a small fraction of rods fail to untwist as the two rod interface forms and act as mobile pinning centers. Capillary flow of short-thick rods drives all the pinning centers to a single location in the composite membrane which now acts like a junction. This pinning junction inhibits complete engulfing of one membrane by the other. Repeated sequential events like this then lead to formation of multiple junctions and the overall cyclic polygon topology. We find that pinning junctions are weakly cross-linked in nature instead of being topological defects. We outline the necessary and sufficient constraints on the nature of rods to obtain stable out of equilibrium cyclic polygon membranes. Our results show a unique counter-intuitive scenario where defects lead to self-assembly of ordered structures.

Contact Interaction of Two Oil Lenses Floating on Surface of Deionized Water

Droplets on the surface of liquid play an important role in a variety of areas, including the petroleum industry, pollution control, and environmental processes. In this work, we study the contact interaction between two floating oil lenses on the surface of immiscible water. The contact interaction between the two floating oil lenses can be divided into three different regimes: (a) the collision involving deformation for low-viscous oils, (b) the direct coalescence for high-viscous oils, and (c) the coexistence (noncoalescence) of oil lenses at relatively high temperatures. The temperature dependence of the coalescence time for the coalescence of two silicone-oil lenses of large viscosities follows the Arrhenius equation.

Evolution and Disappearance of Solvent Drops on Miscible Polymer Subphases

Traditionally, an interface is defined as a boundary between immiscible phases. However, previous work has shown that even when two fluids are completely miscible, they maintain a detectable "effective interface" for long times. Miscible interfaces have been studied in various systems of two fluids with a single boundary between them. However, this work has not extended to the three-phase system of a fluid droplet placed on top of a miscible pool. We show that these three-phase systems obey the same wetting conditions as immiscible systems, and that their drop shapes obey the Augmented Young-Laplace Equation. Over time, the miscible interface diffuses and the shape of the drop evolves. We place 2-microliter drops of water atop miscible poly(acrylamide) solutions. The drop is completely wetted by the subphase, and then remains detectable beneath the surface for many minutes. An initial effective interfacial tension can be approximated to be on the order of 0.5 mN/m using the capillary number. Water and poly(acrylamide) are completely miscible in all concentrations, and yet, when viewed from the side, the drop maintains a capillary shape. Study of this behavior is important to the understanding of effective interfaces between miscible polymer phases, which are pervasive in nature.

Electrostatic interactions between particles through heterogeneous fluid phases

We investigated the electrostatic interactions between particles acting through heterogeneous fluid phases. An oil lens system floating on the surface of water was used to trap particles at different fluid-fluid interfaces. The inner particles are located at the centrosymmetrically curved oil-water interface inside the oil lens while satellite particles are located at the curved air-water interface, separated by a particular distance from the triple phase boundary. The satellite particles are likely to be captured in an energy minimum state due to electrostatic repulsions by the inner particles balanced with the gravity-induced potential energy. As the size of the oil lens decreases upon evaporation, the satellite particles escape from the gravitational confinement at a critical moment. The self-potential values of the inner particles and the satellite particles were calculated by employing an energy balance and the experimentally obtained geometric parameter values. It was found that the self-potential values of the inner particles decrease as oil evaporates over time and that the magnitude of the self-potential of the satellite particles is a hundred times larger than that of the inner particles. These results demonstrate significant effects of the thickness and shape of the nonpolar superphase on the electrostatic interactions between the particles trapped at different fluid-fluid interfaces.

Dynamics of a Water Droplet over a Sessile Oil Droplet: Compound Droplets Satisfying a Neumann Condition

We report the dynamics of compound droplets with a denser liquid (water) droplet over a less dense sessile droplet (mineral oil) that satisfies the Neumann condition. For a fixed size of an oil droplet, depending on the size of the water droplet, either it attains the axisymmetric position or tends to migrate toward the edge of the oil droplet. For a water droplet-to-oil droplet at volume ratio V_w/V_o ≥ 0.05, stable axisymmetric configuration is achieved; for V_w/V_o < 0.05, migration of water droplet is observed. The stability and migration of water droplets of size above and below critical size, respectively, are explained using the force balance at the three-phase contact line and film tension. The larger and smaller droplets that initially attain the axisymmetric position or some radial position, respectively, evaporate continuously and thus migrate toward the edge of the oil droplet. The radial location and migration of the water droplets of different initial sizes with respect to time are studied. Experiments with water droplets on a flat oil-air interface did not show migration, which signified the role of the curved oil-air interface for droplet migration. Finally, coalescence of water droplets of size above the critical size at the axisymmetric position is demonstrated. Our compound droplet studies could be beneficial for applications involving droplet transport where contamination due to direct contact and pinning of droplets on solid surfaces is of concern. Migration and coalescence of water droplets on curved oil-air interfaces could open new frontiers in chemical and biological applications including multiphase processing and biological interaction of cells and atmospheric chemistry.

Evaporation of a Volatile Liquid Lens on the Surface of an Immiscible Liquid

The evaporation behavior of toluene and hexane lenses on the surface of deionized (DI) water is studied. The toluene and hexane lenses during evaporation experience an advancing stage and a receding stage. There exists a significant difference of the evaporation behavior between the toluene lenses and the hexane lenses. The lifetime and largest diameter of both the toluene and hexane lenses increase with increasing the initial volume of the lenses. For the evaporation of the toluene lenses, the lifetime and largest diameter of the lenses decrease with increasing the temperature of DI water. The effect of the residual of the oil molecules on the evaporation of toluene lenses at a temperature of 21 °C is investigated via the evaporation of a series of consecutive toluene lenses being placed on the same position of the surface of DI water. The temporal evolution of the toluene lenses placed after the first toluene lens deviates significantly from that of the first toluene lens. Significant increase of the receding speed occurs at the dimensionless time in a range 0.7-0.8.

Transition Behaviors of Configurations of Colloidal Particles at a Curved Oil-Water Interface

We studied the transition behaviors of colloidal arrangements confined at a centro-symmetrically curved oil-water interface. We found that assemblies composed of several colloidal particles at the curved interface exhibit at least two unique patterns that can be attributed to two factors: heterogeneity of single-colloid self-potential and assembly kinetics. The presence of the two assembly structures indicates that an essential energy barrier between the two structures exists and that one of the structures is kinetically stable. This energy barrier can be overcome via external stimuli (e.g., convection and an optical force), leading to dynamic transitions of the assembly patterns.

Floating mechanism of a small liquid marble

Flotation of small solid objects and liquid droplets on water is critical to natural and industrial activities. This paper reports the floating mechanism of liquid marbles, or liquid droplets coated with hydrophobic microparticles. We used X-ray computed tomography (XCT) to acquire cross-sectional images of the floating liquid marble and interface between the different phases. We then analysed the shape of the liquid marble and the angles at the three-phase contact line (TPCL). We found that the small floating liquid marbles follow the mechanism governing the flotation of solid objects in terms of surface tension forces. However, the contact angles formed and deformation of the liquid marble resemble that of a sessile liquid droplet on a thin, elastic solid. For small liquid marbles, the contact angle varies with volume due to the deformability of the interface.

Self-Pinning on a Liquid Surface

We report on the first experimental evidence of a self-pinning liquid drop on a liquid surface. This particular regime is observed for a miscible heavier oil drop (dichloromethane) deposited on an aqueous solution laden by an ionic surfactant (hexadecyltrimethylammonium bromide). Experimental characterization of the drop shape evolution coupled to particle image velocimetry points to the correlation between the drop profile and the accompanying flow field. A simple model shows that the observed pinned stage is the result of a subtle competition between oil dissolution and surfactant adsorption.

Heterogeneity of single-colloid self-potentials at an oil-water interface

Heterogeneity in the interactions between colloidal particles at an oil-water interface is explored on a single particle level. Such a characteristic arises due to heterogeneity in self-potentials that individual particles possess. Energy minimization is numerically performed to determine the self-potentials of single colloids when the interface-trapped particles form uniquely arranged structures. We demonstrate that the obtained self-potentials correspond to the dipole strength of individual particles at the interface that can be attributed to the generation of abnormally strong and long-range repulsive interactions that should also be heterogeneous. The characterization of self-potentials on a single-particle level can potentially provide insight into the origin of heterogeneity of colloidal suspension systems at multiphasic fluid interfaces. Furthermore, the findings obtained here may facilitate an understanding of the hierarchical relationships associated with scale-dependent colloidal particle behaviors that arise due to interaction heterogeneity.

Effect of interaction heterogeneity on colloidal arrangements at a curved oil-water interface

We report the unique arrangement behaviour of colloidal particles at a curved oil-water interface. Particles trapped at a centrosymmetrically curved oil-water interface, formed by placing an oil lens at a neat air-water interface, organize into diverse arrangement structures due to electrostatic repulsion under the gravitational field. To reveal a possible mechanism behind the observed diversity, we investigate the interactions between pairs of particles at the curved oil-water interface. The magnitude of electrostatic repulsive interactions between pairs of particles is determined by minimizing the total potential of the particle pairs. We show that the pair interactions are quite heterogeneous, following a Gamma distribution. Using the experimentally determined pair potential and the heterogeneity in the potential as input parameters for Monte Carlo simulations, we show that such interaction heterogeneity affects the particle arrangements at the curved interface and results in an observed diversity in the particle arrangement structures. We believe that this work prompts further experimental and simulation studies to extensively understand hierarchical relations from small scale measurements (e.g., pair interactions and heterogeneity) to bulk scale properties (e.g., microstructure and interfacial rheology).

Liquid droplet coalescence and fragmentation at the aqueous-air surface

For hexadecane oil droplets at an aqueous-air surface, the surface film in coexistence with the droplets exhibits two-dimensional gaseous (G), liquid (L), or solid (S) behavior depending upon the temperature and concentration of the cationic surfactant dodecyltrimethylammonium bromide. In the G (L) phase, oil droplets are observed to coalesce (fragment) as a function of time. In the coalescence region, droplets coalesce on all length scales, and the final state is a single oil droplet at the aqueous-air surface. The fragmentation regime is complex. Large oil droplets spread as oil films; hole nucleation breaks up this film into much smaller fluctuating and fragmenting or metastable droplets. Metastable droplets are small contact angle spherical caps and do not fluctuate in time; however, they are unstable over long time periods and eventually sink into the bulk water phase. Buoyancy forces provide a counterbalancing force where the net result is that small oil droplets (radius r < 80 μm) are mostly submerged in the bulk aqueous medium with only a small fraction protruding above the liquid surface. In the G phase, a mechanical stability theory for droplets at liquid surfaces indicates that droplet coalesce is primarily driven by surface tension effects. This theory, which only considers spherical cap shaped surface droplets, qualitatively suggests that in the L phase the sinking of metastable surface droplets into the bulk aqueous medium is driven by a negative line tension and a very small spreading coefficient.

Rapid method for fabricating polymeric biconvex parabolic lenslets

Microlenslets as well as microlens arrays have shown tremendous attractions and successes in miniature optical systems in recent decades. However, the fabrication methods for microlenslets and microlenslet arrays are limited. In this Letter, a rapid and low-cost method for fabricating polymeric biconvex lenslets is presented. This newly developed process is simply based on wetting behavior at interface and is able to produce high-quality biconvex lenslets with controllable size and shape. This technology will greatly simplify the production process and reduce the manufacturing costs for micro-optics.

Stability of a floating water droplet on an oil surface

This article presents a new configuration of a water droplet floating on oil surface. The configuration is characterized by an acute contact angle (i.e., θ2 < π/2). In contrast, the previously identified droplet had an obtuse contact angle, which was easily sunk by a small disturbance. By employing a common surfactant, the new configuration was experimentally verified in a mineral oil with a density similar to that of crude oils. The new droplet is kinetically more stable than the previous configuration and can sustain strong disturbances. The results also highlight the significance of dynamic interfacial adsorption on the stability of the floating droplet.

Equilibrium thickness of large liquid lenses spreading over another liquid surface

This article discusses the equilibrium states and more particularly the equilibrium thickness of large lenses of a liquid spread over the surface of a denser liquid. Both liquids are supposed to be nonvolatile and immiscible. Taking into account the effect of intermolecular forces in addition to the sign of the spreading parameters leads to four possible states. The three first are similar to the states of equilibrium of a liquid spread on a solid surface: total wetting where the floating liquid spreads until it reaches an equilibrium thickness on the order of the molecular size, partial wetting where the floating liquid forms a lens of macroscopic thickness in equilibrium with a "dry" bath, and pseudopartial wetting where the floating liquid spreads as a lens of macroscopic thickness in equilibrium with a thin film covering the bath. The last regime, called pseudototal wetting, consists of a macroscopic lens of the floating liquid covered with a thin film of the bath. These four regimes are described through a free-energy minimization, and their equilibrium thicknesses are predicted. A comparison of this model with experimental results available in the literature and dedicated experiments for the pseudototal wetting state are reported.

Spontaneous particle transport through a triple-fluid phase boundary

We investigate the spontaneous transport of a single particle through an air-water-oil triple phase boundary that is formed by placing a thin oil lens at an air-water interface. We find two distinct transition regimes: a particle initially accelerates upon its adsorption to the air-water-oil triple phase boundary from the air-water interface; subsequently, the particle decelerates after spontaneously detaching from the triple phase boundary. In the first stage, which we call the capillarity regime, the difference in the particle attachment energy to the three fluid-fluid interfaces accounts for the observed initial acceleration. Once it detaches from the air-oil interface and resides solely at the oil-water interface, the particle decelerates due to viscous drag; hence, we call this phase the relaxation regime. We show that the shape of oil lens as well as the size of the particles has a significant influence on the dynamics of particle transport through the triple-fluid phase boundary.

Geometry of the vapor layer under a leidenfrost drop

In the Leidenfrost effect, liquid drops deposited on a hot surface levitate on a thin vapor cushion fed by evaporation of the liquid. This vapor layer forms a concave depression in the drop interface. Using laser-light interference coupled to high-speed imaging, we measured the radius, curvature, and height of the vapor pocket, as well as nonaxisymmetric fluctuations of the interface for water drops at different temperatures. The geometry of the vapor pocket depends primarily on the drop size and not on the substrate temperature.

Can water float on oil?

The floatability of water on oil surface was studied. A numerical model was developed from the Young-Laplace equation on three interfaces (water/oil, water/air, and oil/air) to predict the theoretical equilibration conditions. The model was verified successfully with an oil/water system. The stability of the floating droplet depends on the combination of three interface tensions, oil density, and water droplet volume. For practical purposes, however, the equilibrium contact angle has to be greater than 5° so the water droplet can effectively float. This result has significant applications for biodegrading oil wastes.