Role of dimensionality and axisymmetry in fluid pinch-off and coalescence

Coalescence of biphasic droplets embedded in free standing smectic A films

We investigate micrometer-sized flat droplets consisting of an isotropic core surrounded by a nematic rim in freely suspended smectic A liquid-crystal films. In contrast to purely isotropic droplets which are characterized by a sharp edge and no long-range interactions, the nematic fringe introduces a continuous film thickness change resulting in long-range mutual attraction of droplets. The coalescence scenario is divided in two phases. The first one consists in the fusion of the nematic regions. The second phase involves the dissolution of a thin nematic film between the two isotropic cores. The latter has many similarities with the rupture of thin liquid films between droplets coalescing in an immiscible viscous liquid.

A Review on the Coalescence of Confined Drops with a Focus on Scaling Laws for the Growth of the Liquid Bridge

The surface-tension-driven coalescence of drops has been extensively studied because of the omnipresence of the phenomenon and its significance in various natural and engineering systems. When two drops come into contact, a liquid bridge is formed between them and then grows in its lateral dimensions. As a result, the two drops merge to become a bigger drop. The growth dynamics of the bridge are governed by a balance between the driving force and the viscous and inertial resistances of involved liquids, and it is usually represented by power-law scaling relations on the temporal evolution of the bridge dimension. Such scaling laws have been well-characterized for the coalescence of unconfined or freely suspended drops. However, drops are often confined by solid or liquid surfaces and thus are a different shape from spheres, which affects their coalescence dynamics. As such, the coalescence of confined drops poses more complicated interfacial fluid dynamics challenges compared to that of unconfined drops. Although there have been several studies on the coalescence of confined drops, they have not been systematically reviewed in terms of the properties and geometry of the confining surface. Thus, we aim to review the current literature on the coalescence of confined drops in three categories: drop coalescence on a solid surface, drop coalescence on a deformable surface, and drop coalescence between two parallel surfaces with a small gap (i.e., Hele-Shaw cell), with a focus on power-law scaling relations, and to suggest challenges and outlooks for future research on the phenomena.

Numerical Study of the Coalescence and Mixing of Drops of Different Polymeric Materials

In this study, we employ direct numerical simulation (DNS) to investigate the solutal hydrodynamics dictating the three-dimensional coalescence of microscopic, identical-sized sessile drops of different but miscible shear-thinning polymeric liquids (namely, PVAc or polyvinyl acetate and PMMA or polymethylmethacrylate), with the drops being in partially wetted configuration. Despite the ubiquitousness of the interaction of different dissimilar droplets in a variety of engineering problems ranging from additive manufacturing to understanding the behavior of photonic crystals, coalescence of drops composed of different polymeric and non-Newtonian materials has not been significantly explored. Interaction of such dissimilar droplets often involves simultaneous drop spreading, coalescence, and mixing. The mixing dynamics of the dissimilar drops are governed by interphase diffusion, the residual kinetic energy of the drops stemming from the fact that coalescence starts before the spreading of the drops have been completed, and the solutal Marangoni convection. We provide the three-dimensional velocity fields and velocity vectors inside the completely miscible, dissimilar coalescing droplets. Our simulations explicate the relative influence of these different effects in determining the flow field at different locations and at different time instances and the consequent mixing behavior inside the interacting drops. We also show the non-monotonic (in terms of the direction of migration) propagation of the mixing front of the miscible coalescing drops over time. We also establish that the overall mixing (on either side of the mixing front) speeds up as the Marangoni effects dictate the mixing. We anticipate that our study will provide an important foundation for studying miscible multi-material liquid systems, which will be crucial for applications such as inkjet or aerosol jet printing, lab-on-a-chip, polymer processing, etc.

Nonuniversal Self-Similarity for Jump-to-Contact Dynamics between Viscous Drops under van der Waals Attraction

When two small viscous drops are sufficiently close, van der Waals force overcomes surface tension and deforms the surfaces into contact, initiating coalescence. The dynamics of surface deformation across an inviscid gap are self-similar as contact is approached, with both radial and gap scales varying as t^{'1/3} for time until contact t^{'}. Van der Waals and viscous forces are dominant. The self-similar profiles are both nonuniversal and of the second kind: the observed t^{'1/3} behavior is selected only by the subdominant surface tension.

Viscoelastic necking dynamics between attractive microgels

HYPOTHESIS

Microgels can deform and interpenetrate and display colloid/polymer duality. The effective interaction of microgels in the collapsed state is governed by the interplay of polymer-solvent interfacial tension and bulk elasticity. A connecting neck is shown to mediate microgel interaction, but its temporal evolution has not been addressed. We hypothesize that the necking dynamics of attractive microgels exhibits liquid-like or solid-like behavior over different time and length scales.

EXPERIMENTS

We simulate the merging and pinching of attractive microgels with different crosslinking densities in explicit solvent using dissipative particle dynamics. The temporal coalescence dynamics of microgels is investigated and compared with simple liquid and polymeric droplets. We model the neck growth on long time scales using Maxwell model of polymer relaxation and compare the theoretical prediction with simulation data. The mechanical strength of the neck is characterized systematically via simulated pinch-off of microgels by steered molecular dynamics.

FINDINGS

We evidence a crossover in the coalescence dynamics reflecting the viscoelastic signature of microgels. In contrast to the common knowledge that viscoelastic materials respond elastically on short time scales, the early expansion of the microgel neck exhibits a linear behavior, similar to the viscous coalescence of liquid droplets. However, the late regime with arrested dynamics resembles sintering of solid particles. Through an analytical model relating microgel dynamics to neck growth, we show that the long-term behavior is governed by stress relaxation of the polymers in the neck region and predict an exponential decay in the rate of growth, which agrees favorably with the simulation. Different from coalescence, the thread thinning in microgel breakup primarily highlights its polymeric characteristics.

Quasi-two-dimensional coalescence of nematic and isotropic droplets and Rayleigh-Plateau instability in flat optical cells

We investigated the coalescence of nematic droplets in an isotropic environment and that of isotropic droplets in a nematic environment in quasi-two-dimensional geometry of a flat optical cell. Two different regimes of coalescence were found. In the circular meniscus between the nematic and isotropic regions both nematic and isotropic phases exist. As a result, two bridges form at coalescence: a nematic and an isotropic bridge. In this work, we focus on the situation when nematic wets the cell surface. The coalescence of nematic droplets starts near the cell surfaces where the droplet bridge from the nematic phase is formed. An outer bridge connecting the isotropic environment is localized in the middle of the cell. When the outer bridge gets thinner it becomes unstable and breaks up. A series of pinch-offs leads to the formation of satellite droplets. On the contrary, when isotropic droplets coalesce, the coalescence starts in the middle of the cell and breaking of the bridges occurs without instability and without the formation of satellite droplets. Breakup of the outer bridge is a new example of Rayleigh-Plateau instability in addition to actively studied transformation and breaking of filaments and stretched droplets.

Phase diagram and snap-off transition for twisted party balloons

Many of us have the experience of inflating balloons and twisting them into different shapes and animals. Snapping the balloon into two separate compartments is a necessary step that bears resemblance to the pinch-off phenomenon when a water droplet detaches from the faucet. In addition to testing whether balloons exhibit the properties of self-similarity and memory effect that are often associated with the latter event, we determine their phase diagram by experiments. It turns out that a common party balloon does not just snap, but can assume five more shapes, i.e., straight, necking, wrinkled, helix, and supercoil, depending on the twist angle and ratio of its length and diameter. Moreover, history also matters due to their prominent hysteresis. One may shift the phase boundary and/or reshuffle the phases by untwisting or lengthening the balloon at different twist angle and initial length. A heuristic minimal model is provided to obtain analytic expressions for the phase boundaries.

Coalescence of Microscopic Polymeric Drops: Effect of Drop Impact Velocities

In this paper, we employ the direct numerical simulation (DNS) method for probing three-dimensional, axisymmetric coalescence of microscale, power-law-obeying, and shear-thinning polymeric liquid drops of identical sizes impacting a solid, solvophilic substrate with a finite velocity. Unlike the cases of drop coalescence of Newtonian liquid drops, coalescence of non-Newtonian polymeric drops has received very little attention. Our study bridges this gap by providing (1) the time-dependent, three-dimensional (3D) velocity field and 3D velocity vectors inside two coalescing polymeric drops in the presence of a solid substrate and (2) the effect of the drop impact velocity (on the solid substrate), quantified by the Weber number (We), on the coalescence dynamics. Our simulations reveal that the drop coalescence is qualitatively similar for different We values, although the velocity magnitudes involved, the time required to attain different stages of coalescence, and the time needed to attain equilibrium vary drastically for finitely large We values. Finally, we provide detailed simulation-based, as well as physics-based, scaling laws describing the growth of the height and the width of the bridge (formed due to coalescence) dictating the 3D coalescence event. Our analyses reveal distinct scaling laws for the growth of bridge height and width for early and late stages of coalescence as a function of We. We also provide simulation-based coalescence results for the case of two unequal sized drops impacting on a substrate (nonaxisymmetric coalescence) as well as results for axisymmetric coalescence for drops of different rheology. We anticipate that our findings will be critical in better understanding events such as inkjet or aerosol jet polymer printing, dynamics of polymer blends, and many more.

Dynamics of capillary coalescence and breakup: Quasi-two-dimensional nematic and isotropic droplets

We observed the formation of small satellite droplets from the bridge at droplet coalescence. Investigations were made using a Hele-Shaw cell in the two-phase region at the nematic-isotropic phase transition. In previous works on coalescence it was considered that before the start of coalescence there exists a bridge between the outer fluid connecting regions on the two sides of the droplets (outer bridge). After the start of coalescence, a bridge connecting the two droplets appears (droplet bridge) and the outer bridge is broken. We have shown that there are coalescence processes where after the start of coalescence both the droplet bridge and the outer bridge can exist. This cardinally changes the coalescence process. During the first coalescence stage the size of the outer bridge decreases and the size of the droplet bridge increases. During the second stage the outer bridge becomes unstable which leads to pinch-off, formation of pointed end domains, secondary instability, splitting of pointed end domains, and formation of satellite droplets. We found the linear dependence of the minimum bridge radius on time near bridge breakup. This behavior confirms the capillary viscous regime of bridge breakup. Our work connects two areas of fluid dynamics: coalescence and breakup with formation of satellite droplets.

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.

Coalescence of isotropic droplets in overheated free standing smectic films

A theoretical study of the interaction and coalescence of isotropic droplets in overheated free-standing smectic films (FSSF) is presented. Experimentally it is clear that merging of such droplets is extremely rare. On the basis of the general thermodynamic approach to the stability of FSSF, we determined the energy gains and losses involved in the coalescence process. The main contributions to the critical work of drop coalescence are due to the gain related to the decrease of the surface energy of the merging drops, which is opposed by the entropic repulsions of elementary steps at the smectic interface between them. To quantify the evolution of the merging drops, we use a simple geometrical model in which the volume of the smectic material, rearranged in the process of coalescence, is described by an asymmetrical pyramid at the intersection of two drops. In this way, the critical work for drop coalescence and the corresponding energy barrier have been calculated. The probability of the thermal activation of the coalescence process was found to be negligibly small, indicating that droplet merging can be initiated by only an external stimulus. The dynamics of drop merging was calculated by equating the capillary force driving the coalescence, and the Stokes viscous force slowing it down. For the latter, an approximation of moving oblate spheroids permitting exact calculations was used. The time evolution of the height of the neck between the coalescing drops and that of their lateral size are in good agreement with experiments.

Self similarity of liquid droplet coalescence in a quasi-2D free-standing liquid-crystal film

Coalescence of droplets is an ubiquitous phenomenon in chemical, physical and biological systems. The process of merging of liquid objects has been studied during the past years experimentally and theoretically in different geometries. We introduce a unique system that allows a quasi two-dimensional description of the coalescence process: Micrometer-sized flat droplets in freely suspended smectic liquid-crystal films. We find that the bridge connecting the droplets grows linearly in time during the initial stage of coalescence, both with respect to its height and lateral width. We also verify self-similar dynamics of the bridge during the first stage of coalescence. We compare our results with a model based on the thin sheet equations. Our experiments confirm that the most important geometrical parameter influencing the coalescence rate is the contact angle of the droplets.

Self-Similar Liquid Lens Coalescence

A basic feature of liquid drops is that they can merge upon contact to form a larger drop. In spite of its importance to various applications, drop coalescence on prewetted substrates has received little attention. Here, we experimentally and theoretically reveal the dynamics of drop coalescence on a thick layer of a low viscosity liquid. It is shown that these so-called "liquid lenses" merge by the self-similar vertical growth of a bridge connecting the two lenses. Using a slender analysis, we derive similarity solutions corresponding to the viscous and inertial limits. Excellent agreement is found with the experiments without any adjustable parameters, capturing both the spatial and temporal structures of the flow during coalescence. Finally, we consider the crossover between the two regimes and show that all data of different lens viscosities collapse on a single curve capturing the full range of the coalescence dynamics.

Universality in the viscous-to-inertial coalescence of liquid droplets

We present a theory on the coalescence of 2 spherical liquid droplets that are initially stationary. The evolution of the radius of a liquid neck formed upon coalescence was formulated as an initial value problem and then solved to yield an exact solution without free parameters, with its 2 asymptotic approximations reproducing the well-known scaling relations in the inertially limited viscous and inertial regimes. The viscous-to-inertial crossover observed in previous research is also recovered by the theory, rendering the collapse of data of different viscosities onto a single curve.

Restoring universality to the pinch-off of a bubble

The pinch-off of a bubble is an example of the formation of a singularity, exhibiting a characteristic separation of length and time scales. Because of this scale separation, one expects universal dynamics that collapse into self-similar behavior determined by the relative importance of viscous, inertial, and capillary forces. Surprisingly, however, the pinch-off of a bubble in a large tank of viscous liquid is known to be nonuniversal. Here, we show that the pinch-off dynamics of a bubble confined in a capillary tube undergo a sequence of two distinct self-similar regimes, even though the entire evolution is controlled by a balance between viscous and capillary forces. We demonstrate that the early-time self-similar regime restores universality to bubble pinch-off by erasing the system's memory of the initial conditions. Our findings have important implications for bubble/drop generation in microfluidic devices, with applications in inkjet printing, medical imaging, and synthesis of particulate materials.

Droplet Coalescence is Initiated by Thermal Motion

The classical notion of the coalescence of two droplets of the same radius R is that surface tension drives an initially singular flow. In this Letter we show, using molecular dynamics simulations of coalescing water nanodroplets, that after single or multiple bridges form due to the presence of thermal capillary waves, the bridge growth commences in a thermal regime. Here, the bridges expand linearly in time much faster than the viscous-capillary speed due to collective molecular jumps near the bridge fronts. Transition to the classical hydrodynamic regime only occurs once the bridge radius exceeds a thermal length scale l_{T}∼sqrt[R].

Viscous dynamics of drops and bubbles in Hele-Shaw cells: Drainage, drag friction, coalescence, and bursting

In this review article, we discuss recent studies on drops and bubbles in Hele-Shaw cells, focusing on how scaling laws exhibit crossovers from the three-dimensional counterparts and focusing on topics in which viscosity plays an important role. By virtue of progresses in analytical theory and high-speed imaging, dynamics of drops and bubbles have actively been studied with the aid of scaling arguments. However, compared with three-dimensional problems, studies on the corresponding problems in Hele-Shaw cells are still limited. This review demonstrates that the effect of confinement in the Hele-Shaw cell introduces new physics allowing different scaling regimes to appear. For this purpose, we discuss various examples that are potentially important for industrial applications handling drops and bubbles in confined spaces by showing agreement between experiments and scaling theories. As a result, this review provides a collection of problems in hydrodynamics that may be analytically solved or that may be worth studying numerically in the near future.

Surfactant effects on the coalescence of a drop in a Hele-Shaw cell

In this work the coalescence of an aqueous drop with a flat aqueous-organic interface was investigated in a thin gap Hele-Shaw cell. Different concentrations of a nonionic surfactant (Span 80) dissolved in the organic phase were studied. We present experimental results on the velocity field inside a coalescing droplet in the presence of surfactants. The evolution of the neck between the drop and the interface was studied with high-speed imaging. It was found that the time evolution of the neck at the initial stages of coalescence follows a linear trend, which suggests that the local surfactant concentration at the neck region for this stage of coalescence can be considered quasiconstant in time. This neck expansion can be described by the linear law developed for pure systems when the surfactant concentration at the neck is assumed higher than in the bulk solution. In addition, velocity and vorticity fields were computed inside the coalescing droplet and the bulk homophase using a high-speed shadowgraphy technique. The significant wall effects in the Hele-Shaw cell in the transverse axis cause the two vertical velocity components towards the singularity rupture point, from the drop and from the bulk homophase, to be of the same order of magnitude. This movement together with the neck expansion creates two pairs of counteracting vortices in the drop and in the bulk phase. The neck velocity is the average of the advection velocities of the two counteracting vortex pairs on each side of the neck. The presence of the surfactant slows down the dynamics of the coalescence, affects the propagation direction of the pair of vortices in the bulk phase, and reduces their size faster compared to the system without surfactant.

Approach and coalescence of liquid drops in air

The coalescence of liquid drops has conventionally been thought to have just two regimes when the drops are brought together slowly in vacuum or air: a viscous regime corresponding to the Stokes-flow limit and a later inertially dominated regime. Recent work [Proc. Natl. Acad. Sci. 109, 6857 (2012)] found that the Stokes-flow limit cannot be reached in the early moments of coalescence, because the inertia of the drops cannot be neglected then. Instead, the drops are described by an "inertially limited viscous" regime, where surface tension, inertia, and viscous forces all balance. The dynamics continue in this regime until either viscosity or inertia dominate on their own. I use an ultrafast electrical method and high-speed imaging to provide a detailed description of coalescence near the moment of contact for drops that approach at low speed and coalesce as undeformed spheres. These measurements support a description of coalescence having three regimes. Signatures both before and after contact identify a threshold approach speed for deformation of the drops by the ambient gas.

Scaling behavior of universal pinch-off in two-dimensional foam

We study the power-law scaling behavior and pinch-off morphology of two-dimensional bubble rafts under tension. As a function of pulling speed, we observe two distinct pinch-off morphologies that have been observed in other fluid systems: long threads (LT) and double-cone (DC). At any given pulling speed, there is a nonzero probability of observing LT or DC, with the probability of observing LT modes increasing with pulling velocity. The bubble rafts are composed of millimeter scale bubbles, and we are able to directly observe pinch-off to the point of final separation and measure the scaling of the minimum width in time. For both the LT and DC modes, the final scaling regime before pinch-off exhibits a universal power-law scaling behavior, with power-law fitting exponents of 0.73 ± 0.01. However, the final cone angle is different for states that initially exhibit LT or DC pinch-off, and for the LT case, the final scaling is best described as a local double-cone mode.

Imbibition of a textured surface decorated by short pillars with rounded edges

Imbibition of micropatterned surfaces can have broad technological and fundamental implications for areas ranging from biomedical devices and fuel transport to writing with ink. Despite rapidly growing interests aimed at various applications, a fundamental physical understanding of the imbibition dynamics is still in its infancy. Recently, two simple scaling regimes for the dynamics have been established for a textured surface decorated with long pillars whose top and bottom edges are sharp. Here, we study the imbibition dynamics of textured surfaces decorated by short pillars with rounded edges, to find a different scaling regime. Interestingly, this regime originates not from the balance of two effects but from the hybrid balance of three effects. Furthermore, this scaling law can be universal or independent of the details of the texture geometry. We envision that this potentially universal scaling regime might be ubiquitous and will be useful in the handling and transportation of a small amount of liquid.

Coalescence preference depends on size inequality

During bubble or droplet coalescence, there is a puzzling tendency for the coalesced bubble or droplet to be preferentially placed closer to the larger of its two parents. We confirm that this preference is a function of parent size ratio by directly visualizing coalescing air bubbles on an oil-water interface and coalescing water droplets immersed in oil. We find that the final position of the coalesced sphere is controlled by surface energy release and is related to the parent size ratio by a power-law relationship. 

Initial spreading of low-viscosity drops on partially wetting surfaces

Liquid drops start spreading directly after coming into contact with a partially wetting substrate. Although this phenomenon involves a three-phase contact line, the spreading motion is very fast. We study the initial spreading dynamics of low-viscosity drops using two complementary methods: molecular dynamics simulations and high-speed imaging. We access previously unexplored length and time scales and provide a detailed picture on how the initial contact between the liquid drop and the solid is established. Both methods unambiguously point toward a spreading regime that is independent of wettability, with the contact radius growing as the square root of time.

The inexorable resistance of inertia determines the initial regime of drop coalescence

Drop coalescence is central to diverse processes involving dispersions of drops in industrial, engineering, and scientific realms. During coalescence, two drops first touch and then merge as the liquid neck connecting them grows from initially microscopic scales to a size comparable to the drop diameters. The curvature of the interface is infinite at the point where the drops first make contact, and the flows that ensue as the two drops coalesce are intimately coupled to this singularity in the dynamics. Conventionally, this process has been thought to have just two dynamical regimes: a viscous and an inertial regime with a cross-over region between them. We use experiments and simulations to reveal that a third regime, one that describes the initial dynamics of coalescence for all drop viscosities, has been missed. An argument based on force balance allows the construction of a new coalescence phase diagram.

Dimensional crossover in the coalescence dynamics of viscous drops confined in between two plates

Coalescence of liquid drops is a daily phenomenon familiar to everybody and is related to many fields from biology to astronomy and also related to a variety of practical problems in industry. However, the detailed physical understanding of the dynamics has been revealed only recently with the aid of high-speed camera, high-performance computer, and scaling analysis. In this study, coalescence of a viscous drop to a bath of the same liquid is studied in a confined space. This is because dealing with a small amount of liquid drops becomes increasingly important (e.g., in industrial and biological applications). Here, the aqueous drop and bath are surrounded by low-viscosity oil and sandwiched by two parallel plates of the cell. We quantify experimentally the width of a neck that bridges the drop and the bath during coalescence. As a result, we find that the neck width increases linearly with time at short times, but the dynamics slows down significantly at longer times. Thanks to simple and original scaling arguments, we clearly show that this transition of the dynamics with time in a single coalescence event is brought about by a crossover from a three-dimensional viscous dynamics for a spherical drop to a quasi two-dimensional one for a disk drop. In addition, we report an unusual type of coalescence that is possibly caused by naturally accumulated electric charge in the confined geometry and whose dynamics seems self-similar.

Viscous to inertial crossover in liquid drop coalescence

Using an electrical method and high-speed imaging, we probe drop coalescence down to 10 ns after the drops touch. By varying the liquid viscosity over two decades, we conclude that, at a sufficiently low approach velocity where deformation is not present, the drops coalesce with an unexpectedly late crossover time between a regime dominated by viscous and one dominated by inertial effects. We argue that the late crossover, not accounted for in the theory, can be explained by an appropriate choice of length scales present in the flow geometry.

Experimental and numerical investigation of the equilibrium geometry of liquid lenses

The equilibrium configuration of a nonwetted three fluid system takes the form of a floating liquid lens, where the lens resides between an upper and lower phase. The axisymmetric profiles of the three interfaces can be computed by solving the nonlinear Young-Laplace differential equation for each interface with coupled boundary conditions at the contact line. Here we describe a numerical method applicable to sessile or pendant lenses and provide a free, downloadable Mathematica Player file which uses a graphical interface for analyzing and plotting lens profiles. The results of the calculations were compared to optical photographs of various liquid lens systems which were analyzed using basic ray-tracing and Moiré imaging. The lens profile calculator, together with a measurement of the lens radius for a known volume, provides a simple and convenient method of determining the spreading coefficient (S) of a liquid lens system if all other fluid parameters are known. If surfactants are present, the subphase surface tension must also be self-consistently determined. A procedure is described for extracting characteristic features in the optical images to uniquely determine both parameters. The method gave good agreement with literature values for pure fluids such as alkanes on water and also for systems with a surfactant (hexadecane/DTAB), which show a transition from partial wetting to the pseudopartial wetting regime. Our technique is the analog of axisymmetric drop shape analysis, applied to a three fluid system.

Coalescence driven by line tension in thin nematic films

Thin nematic films deposited on liquid substrates provide a unique situation to investigate coalescence: the whole process can be followed under microscope over a wide range of times, and temperature allows us to monitor the surface viscosity of the surrounding fluid. For the first time, the complete scenario of 2D coalescence has been recorded for a given system in both inviscid limit and viscous environment, enabling us to identify the successive routes of dissipation. In particular, 2D "viscous bubbles" of the surrounding viscous fluid with a bulbous shape formed in the gap between coalescing films are observed. Available models are adapted to our specific case and account satisfactorily for the whole process.

Some specificities of wetting by cyanobiphenyl liquid crystals

The present paper provides an up to date restatement of the wetting behaviour of the series of cyanobiphenyl liquid crystals (LCs) on usual substrates, i.e. oxidized silicon wafers, water and glycerol, at both the macroscopic and microscopic scale, in the nematic range of temperature. We show that on water the systems are close to a wetting transition, especially 5CB and 7CB. In that case, the wetting behaviour is controlled by the presence of impurities. On a mesoscopic scale, we observe for all our (thin LC film-substrate) systems an identical, complex, but well defined general scenario, not accounted for by the available models. In the last part, we present a study on line tension which results from the specific organization of LCs at the edge of the nematic film. We report preliminary results on two-dimensional film coalescence where this line tension plays a major role.