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The numerical results from the modeling of liquid crystals dispersed in aqueous solutions in the form of axially symmetric droplets, with the aim of helping to facilitate the development of liquid crystal biosensors, were obtained. We developed a transient two-dimensional nonlinear model obtained via torque balance that incorporates Frank's elastic free energy. In order to perform parametric studies, we defined the scaled parameters based on the surface viscosity and the homeotropic anchoring energy at the droplet interface. To evaluate the performance of the biosensor, the average angle and characteristic time were defined as performance criteria. Using these results, we studied the bulk reorientation of liquid crystal droplets in aqueous solutions caused by biomolecular interaction. Furthermore, we examined how surface viscosity affects the performance of a biosensor in the case of weak planar anchoring. The droplet interface ordering was modeled using the Euler–Lagrange equation. The droplets' equilibrium was determined by minimizing their total distortion energy based on the interaction between their surface and bulk elastic energy. Two factors that contributed to the biosensor performance were homeotropic strength and surface viscosity. This highlights the importance of controlling the surface and physicochemical properties to achieve the desired liquid crystal orientation. In addition, our results provide insight into the role that surface viscosity plays in controlling radial configuration. [ABSTRACT FROM AUTHOR]

The numerical results from the modeling of liquid crystals dispersed in aqueous solutions in the form of axially symmetric droplets, with the aim of helping to facilitate the development of liquid crystal biosensors, were obtained. We developed a transient two-dimensional nonlinear model obtained via torque balance that incorporates Frank’s elastic free energy. In order to perform parametric studies, we defined the scaled parameters based on the surface viscosity and the homeotropic anchoring energy at the droplet interface. To evaluate the performance of the biosensor, the average angle and characteristic time were defined as performance criteria. Using these results, we studied the bulk reorientation of liquid crystal droplets in aqueous solutions caused by biomolecular interaction. Furthermore, we examined how surface viscosity affects the performance of a biosensor in the case of weak planar anchoring. The droplet interface ordering was modeled using the Euler–Lagrange equation. The droplets’ equilibrium was determined by minimizing their total distortion energy based on the interaction between their surface and bulk elastic energy. Two factors that contributed to the biosensor performance were homeotropic strength and surface viscosity. This highlights the importance of controlling the surface and physicochemical properties to achieve the desired liquid crystal orientation. In addition, our results provide insight into the role that surface viscosity plays in controlling radial configuration.

Dynamic extracellular environments profoundly affect the behavior and function of cells both biochemically and mechanically. Neurite initiation is the first step for neurons to establish intricate neuronal networks. How such a process is modulated by mechanical factors is not fully understood. Particularly, it is unknown whether the molecular clutch model, which has been used to explain cell responses to matrix rigidity, also holds for neurite initiation. To study how mechanical properties modulate neurite initiation, substrates with various well‐defined surface viscosities using supported lipid bilayers (SLBs) are synthesized. The results show that ligands with intermediate viscosity greatly maximize neurite initiation in primary neurons, while neurite initiation is drastically limited on substrates with higher or lower viscosity. Importantly, biochemical characterizations reveal altered focal adhesion and calpain activity are associated with distinct neurite initiation patterns. Collectively, these results indicate that neurite initiation is surface viscosity‐dependent; there is an optimal range of surface viscosities to drive neurite initiation. Upon binding to ligands of varying viscosities, calpain activity is differentially triggered and leads to distinct levels of neurite outgrowth. These findings not only enhance the understanding of how extracellular environments regulate neurons, but also demonstrate the potential utility of SLBs for neural tissue engineering applications. [ABSTRACT FROM AUTHOR]

Copyright of Modern Food Science & Technology is the property of Editorial Office of Modern Food Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The mechanical behaviour of the oxide film that develops at the metal-air interface of molten A356 aluminum alloy is investigated in relation to the oxidation level. Based on two previous papers -- the first one focusing on the chemical aspects of the oxidation process at the liquid surface of molten aluminum, the second one focusing on its mechanical characterization, the present paper aims at reconciling these two approaches usually considered separately in the literature and proposes a stand-alone mechano-chemical model. The oxidation process is first characterized from thermogravimetric analyses together with scanning electron microscopy imaging. Then the mechanical properties of the oxide layer are determined at a given oxidation level, making use of a new annular MHD surface viscometer placed inside a chemical reactor. The kinetic study delivers a better understanding of the oxidation process involved at the surface of the A356 alloy and leads to the development of a revised version of the Deal--Grove model. In addition, the mechanical study reveals the highly viscous and shear-thinning behaviours of the oxide layer. [ABSTRACT FROM AUTHOR]

Wrinkling instabilities of thin elastic sheets can be used to generate periodic structures over a wide range of length scales. Viscosity of the thin elastic sheet or its surrounding medium has been shown to be responsible for dynamic processes. We here consider wrinkling of fluid deformable surfaces. In contrast with thin elastic sheets, with in-plane and out-of-plane elasticity, these surfaces are characterized by in-plane viscous flow and out-of-plane elasticity and have been established as model systems for biomembranes and cellular sheets. We use this hydrodynamic theory and numerically explore the formation of wrinkles and their coarsening, either by a continuous reduction of the enclosed volume or by the continuous increase of the surface area. Both lead to almost identical results for wrinkle formation and the coarsening process, for which a scaling law for the wavenumber is obtained for a broad range of surface viscosity and rate of change of volume or area. However, for large Reynolds numbers and small changes in volume or area, wrinkling can be suppressed and surface hydrodynamics allows for global shape changes following the minimal energy configurations of the Helfrich energy for corresponding reduced volumes.

This paper discusses the momentum jump condition across a viscous interface, which shows Newtonian behavior, i.e., is a Boussinesq surface fluid, by reviewing and expanding the works of Edwards et al. (1991) and Slattery et al. (2007). The necessary geometrical/mathematical tools for the derivation of the jump condition, and the jump condition itself are systematically derived for different cases defined based on the functional form of the surfaces. The momentum jump condition for interfaces with various degrees of deformability are presented both for arbitrary coordinate systems and explicitly in the Cartesian, the cylindrical and the spherical coordinates. Finally, the jump condition is simplified for thin rectangular and cylindrical films, and the contribution of the surface viscosities in the thin film limit is discussed. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Henan University of Technology Natural Science Edition is the property of Henan University of Technology Journal Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Foams age as gas diffuses from smaller bubbles to larger ones. This results in an increase of the average bubble size, which leads to a modification of the foam rheology and overall stability. This is why understanding coarsening is crucial for the creation of aerated materials. The coarsening rate is affected by various mechanisms, which are impacted by geometry (bubble size and bubble size distribution), liquid fraction, air solubility in the liquid phase, permeability of the interfaces, mechanical properties of the liquid phase and of the interfaces. In many real systems changing a single parameter will impact several mechanisms. In this work, we use a two-bubble experiment to isolate and explore the impact of bulk and surface viscosity on the rate of bubble coarsening. The experimental set-up allows us to decouple the impact of viscous dissipation from the rate of gas diffusion. We build a model in which the gas transfer is driven by Laplace pressure differences and hindered by viscous stresses. We need to take into account gravitational stresses, and dissipation in the connections, with which our model is fully predictive of the experimental data. [Display omitted] [ABSTRACT FROM AUTHOR]

An antibubble is a liquid droplet wrapped by a thin layer of gas, inside a bulk liquid usually of the same composition. The lifetime of an antibubble is governed by the drainage of the gas between the two liquid-gas interfaces populated by surfactants. Depending on the relative magnitude of surface viscosity and elastic moduli, which directly depend on or are determined by the nature of surfactants, the lifetime of an antibubble may vary a lot, from few seconds to few minutes. While such a difference can be predicted with models that include the role of interfacial properties, they were not observed experimentally in previous studies, due to important sources of dispersion. In this review, the main sources of dispersion are identified, such as (i) the initial amount of gas embedded in the antibubble, (ii) the level of saturation of gas in the bulk liquid, (iii) the presence of dust particles (<0.5 μm) in the gas, and (iv) three-dimensional flow effects. By accounting for these various effects, we obtain a coherent view on the lifetime of an antibubble, as a function of its radius and the surface rheology, with excellent consistency between experiments and modeling. Results thus demonstrate that controlling the size and lifetime of antibubbles is achievable. Unlabelled Image • Surface elasticity plays a dominant role in antibubble lifetime for small surfactant molecules; • Above a threshold, the lifetime is almost independent on the bulk concentration of surfactants; • Accounting precisely for gas dissolution in the liquid is crucial to obtain qualitative agreement with modeling. • The antibubble lifetime increases with the radius; • In controlled conditions, the antibubble collapse is shown to be a deterministic process; [ABSTRACT FROM AUTHOR]

The surface and interfacial properties of casein-hydrolyzed peptides were evaluated using measurement of surface and interfacial tensions, surface viscosity, dynamic light scattering (DLS), and freeze-fracture transmission electron microscopy (FF-TEM). In this study, high internal oil phase emulsion (HIPE) gels were successfully prepared, using the surface and interfacial properties of casein peptides. The casein peptides exhibited surface and interfacial activities. The estimated critical micelle concentration (CMC) and γCMC values were 3.0 mg/mL and 47.8 mN/m, and the average size of casein peptide micelles was 13.2 ± 1.7 nm. The surface shear viscosity of an aqueous casein peptide solution at 10 mg/mL was 1603 μPa ms, which is fifteen times larger than that of sodium dodecyl sulfate (SDS, 106 μPa ms). The larger surface viscosity of casein peptide adsorbed layer could stabilize emulsions and prevent flocculation and coalescence. High internal oil phase gel emulsions were then prepared by slowly adding oil and polyisobutene into an aqueous casein peptide solution/glycerol mixture with different compositions. Based on the pseudo ternary 15 wt% aqueous casein peptide solution/polyisobutene/glycerol phase diagram, the HIPE containing the maximum 88.1 wt% (91.5 vol%) of oil is obtained by the addition of 0.36 wt% of casein peptides. The use of only a small amount of protein-hydrolyzed peptides instead of the commonly used synthetic surfactants for HIPE preparation has great advantages for the widespread application of HIPE technology. [ABSTRACT FROM AUTHOR]

When surfactants adsorb at liquid interfaces, they not only decrease the surface tension, they confer rheological properties to the interfaces. There are two types of rheological parameters associated to interfacial layers: compression and shear. The elastic response is described by a storage modulus and the dissipation by a loss modulus or equivalently a surface viscosity. Various types of instruments are available for the measurements of these coefficients, the most common being oscillating pendent drops instruments and rheometers equipped with bicones. These instruments are applicable to systems with large enough interfacial tensions, typically above a few mN/m. We use a new type of instrument based on spinning drop oscillations, allowing to extend the interfacial rheology studies to low and ultralow interfacial tension systems. We present examples of measurements with systems of high and low tension, discuss the possible artifacts and demonstrate the capability of this new technique. We emphasize that the data shown for low interfacial tensions are the first reported in the literature. The instrument is potentially interesting for instance in enhanced oil recovery or demulsification studies. [ABSTRACT FROM AUTHOR]

Cell response to matrix rigidity has been explained by the mechanical properties of the actin-talin-integrin-fibronectin clutch. Here the molecular clutch model is extended to account for cell interactions with purely viscous surfaces (i.e., without an elastic component). Supported lipid bilayers present an idealized and controllable system throughwhich to study this concept. Using lipids of different diffusion coefficients, the mobility (i.e., surface viscosity) of the presented ligands (in this case RGD) was altered by an order of magnitude. Cell size and cytoskeletal organization were proportional to viscosity. Furthermore, there was a higher number of focal adhesions and a higher phosphorylation of FAK on less-mobile (more-viscous) surfaces. Actin retrograde flow, an indicator of the force exerted on surfaces, was also seen to be faster on more mobile surfaces. This has consequential effects on downstream molecules; the mechanosensitive YAP protein localized to the nucleus more on less-mobile (more-viscous) surfaces and differentiation of myoblast cells was enhanced on higher viscosity. This behavior was explained within the framework of the molecular clutch model, with lower viscosity leading to a low force loading rate, preventing the exposure of mechanosensitive proteins, and with a higher viscosity causing a higher force loading rate exposing these sites, activating downstream pathways. Consequently, the understanding of how viscosity (regardless of matrix stiffness) influences cell response adds a further tool to engineer materials that control cell behavior. [ABSTRACT FROM AUTHOR]

An improved Smoothed Particle Hydrodynamics (SPH) method is used to study wave overtopping for different coastal structures. Simulated wave overtopping is too sensitive to the particle movements near the free surface boundary; however, the calculated flow acceleration by means of common SPH methods is not free of errors at this boundary due to the truncation of kernel function and contribution of fewer particles in solving the governing equations. In this paper, this problem is solved by modifying the viscosity of surface particles based on the concept of surface viscosity originally introduced by Xu (2010). By means of the introduced modification, unrealistic particle fluctuation at free surface boundary can be decreased significantly while keeping the model accuracy. This improvement can be used for both Incompressible and Weakly Compressible SPH methods and its implementation is easy and computationally efficient too. Different cases including dam break, solitary wave breaking and wave overtopping at vertical and sloping seawalls are simulated with the modified model and the new model is validated via comparing the results with several experimental and numerical data. Based on this study, free surface boundary can be simulated more accurately by means of the introduced modification and as a result, the predicted values particularly the calculated wave-overtopping rate become more reliable. [ABSTRACT FROM AUTHOR]

Polysaccharides have been associated with various biological functions through their binding to membranes, but their specific role is still under debate. The aim of this work was to study the interaction of cationic and anionic polysaccharides with anionic and zwitterionic monolayers, at different subphase compositions, thus analyzing the influence of electrostatics on the interaction. The consequent effect of the polymer-lipid binding on the film properties was studied, with special interest in monolayer dynamics. The results indicate that electrostatic interactions play an important role in polymer-membrane affinity, and that the polymers formed a sub-layer, which increased the shear viscosity of the interface. The interacting polymer, even when it did not penetrate the lipid film, induced a polymer-like behavior of the monolayer regarding its dynamics: the whole film (polymer + lipid) became very viscous. As a consequence, the dynamic of the membrane was affected, thus inducing changes in the film topography, although the energetics for phase transition and the stability of each phase were modified slightly or not at all. [ABSTRACT FROM AUTHOR]

A finite element formulation to approximate the behavior of lipid membranes is proposed. The mathematical model incorporates tangential viscous stresses and bending elastic forces, together with the inextensibility constraint and the enclosed volume constraint. The membrane is discretized by a surface mesh made up of planar triangles, over which a mixed formulation (velocity–curvature) is built based on the viscous bilinear form (Boussinesq–Scriven operator) and the Laplace–Beltrami identity relating position and curvature. A semi-implicit approach is then used to discretize in time, with piecewise linear interpolants for all variables. Two stabilization terms are needed: The first one stabilizes the inextensibility constraint by a pressure–gradient–projection scheme (Codina and Blasco (1997) [33] ), the second couples curvature and velocity to improve temporal stability, as proposed by Bänsch (2001) [36] . The volume constraint is handled by a Lagrange multiplier (which turns out to be the internal pressure), and an analogous strategy is used to filter out rigid-body motions. The nodal positions are updated in a Lagrangian manner according to the velocity solution at each time step. An automatic remeshing strategy maintains suitable refinement and mesh quality throughout the simulation. Numerical experiments show the convergent and robust behavior of the proposed method. Stability limits are obtained from numerous relaxation tests, and convergence with mesh refinement is confirmed both in the relaxation transient and in the final equilibrium shape. Virtual tweezing experiments are also reported, computing the dependence of the deformed membrane shape with the tweezing velocity (a purely dynamical effect). For sufficiently high velocities, a tether develops which shows good agreement, both in its final radius and in its transient behavior, with available analytical solutions. Finally, simulation results of a membrane subject to the simultaneous action of six tweezers illustrate the robustness of the method. [ABSTRACT FROM AUTHOR]

Abstract: An analytical expression for the relaxation time of a nematic liquid crystal is obtained for the first time by considering the influence of surface viscosity, anchoring energy strength and cell gap, validated numerically by using the so-called relaxation method. This general equation for the molecular response time ( ) was derived for a vertical aligned cell and by solving an eigenvalue equation coming from the usual balance of torque equation in the Derzhanskii and Petrov formulation, recovering the usual equations in the appropriate limit. The results show that , where is observed only for strongly anchored cells, while for moderate to weak anchored cells, the exponent lies between 1 and 2, depending on both, surface viscosity and anchoring strength. We found that the surface viscosity is important when calculating the response time, specially for thin cells, critical for liquid crystal devices. The surface viscosity’s effect on the optical response time with pretilt is also explored. Our results bring new insights about the role of surface viscosity and its effects in applied physics. [Copyright &y& Elsevier]

When surfactants adsorb at liquid surfaces, they not only decrease the surface tension, they also confer rheological properties to the surfaces. The most common rheological parameters are the surface compression elasticity and viscosity and the surface shear viscosity. These parameters usually depend on the timescale of the deformation, owing to surface relaxations, and on its amplitude, owing to nonlinear responses. In addition, surfactants can exchange between the bulk and surface, in a way that depends on the amount of bulk surfactant locally available. This complexity explains why the topic has progressed slowly over the years. This review describes the current knowledge, focusing on recent advances, and gives examples of phenomena in which surface rheology plays an important role. [ABSTRACT FROM AUTHOR]

We investigate the director relaxation of a nematic liquid crystal cell by considering the molecular moment of inertia, surface viscosity and anchoring energies at the boundaries. The problem solved here corresponds to a more complete description of the nematic director reorientation, since it incorporates ingredients that are usually neglected due to the mathematical difficulty raised by the moment of inertia and surface viscosity. In order to face this problem, we apply the half space approximation for the sample with the surfaces characterized by a time dependent easy direction, which can be mechanically or optically induced in the system. The results show that both, the molecular moment of inertia and the surface viscosity, have influence on the relaxation process of the director. In particular, the presence of the molecular moment of inertia introduces a finite phase velocity and, consequently, leads to a non-instantaneous response time for the distortion propagation in a liquid crystal cell. [ABSTRACT FROM AUTHOR]

We investigate the relaxation of the director in a nematic liquid crystal cell by considering the presence of different surface viscosities and anchoring energies on the boundaries. To face a more general problem, we also model the surfaces as characterised by time dependent easy directions which can be mechanically or optically induced in the system. In order to connect the analytical results with measurable physical quantities, the temporal behaviour of the optical path difference is established incorporating the contribution of the surface viscosity. [ABSTRACT FROM AUTHOR]

The effect of a localized surface viscosity on the relaxation of a pre-existing deformation in a nematic liquid crystal cell is perturbatively analyzed in the presence of an external field. When the surface viscosity of the system is small enough to use perturbation developments, approximated solutions can be obtained describing the space-time profile of the director angle. It is shown that if the surface viscosity increases, the relaxation becomes slower when compared to the case corresponding to the absence of viscosity. The temporal behavior of the optical path difference is analytically established by incorporating the contribution of the surface viscosity. [ABSTRACT FROM AUTHOR]

The influence of surface viscosity and anchoring energy on the reorientation process of a nematic liquid crystal cell is theoretically investigated. The cell is a slab of thickness, d, whose limiting surfaces are characterised by different anchoring strengths and present easy directions parallel to the bounding surfaces, changing with time due to some external action. The exact space-time profile of the director angle is obtained by means of integral transform techniques and a Green function approach. From this formalism, the time dependence of the optical path difference is exactly determined and its behaviour is analysed in connection with the presence of surface viscosity and different anchoring energies. The problem is also exactly solved in the presence of a constant electric field. It is shown that the compatibility problem between the time derivative of the director field on the surface and in the bulk can be avoided. [ABSTRACT FROM AUTHOR]

The capillary instability of two annular layers confined between two concentric cylinders is considered under conditions of Stokes flow. The inner cylinder, the outer cylinder or both cylinders may be absent, yielding an interior or exterior annular layer or a thread suspended in an infinite medium. The interface exhibits constant surface tension and develops viscous interfacial tensions due to the interfacial flow driven by the instability. A linear stability analysis for axisymmetric perturbations is carried out confirming the stabilizing effect of the surface viscosity over a broad range of conditions. In the case of a thread suspended in vacuum, the surface viscosity is not able to emulate an external medium viscosity. Numerical solutions of a simplified system of evolution equations for long waves using a particle method suggest that surface viscosity prevents thread break-up after a finite evolution time. [ABSTRACT FROM PUBLISHER]

Abstract: The drag force on magnetic microparticles moving along air–liquid interfaces in a magnetic field was determined by trajectory analysis. The drag force in such a system is important for magnetic targeting and retention of drug particles in the airways. For pure, Newtonian liquids, it was shown that the theoretical predictions of drag force found in the literature are consistent with experimental results at low contact angles and high bulk viscosities. As such, these models can be used to predict drag due to bulk viscosity on particles in the airways. In surfactant solutions (water and SDS), it was found that the drag coefficient of the particle varies depending on the surfactant concentration and particle velocity, due to the interplay of particle motion and surfactant concentration. The drag coefficient on the particles when surfactants are present is a function of the particle velocity in addition to the interface properties. This behavior is also significant for drug delivery in the airways, as it would restrict the aggregation of deposited particles, increasing the magnetic field required for particle retention. [Copyright &y& Elsevier]

Abstract: With the goal to identify surface viscosity, this paper proposes the modelling of a shear flow within an annular channel whose floor is put in rotation while its two vertical cylindrical side walls are maintained stationary. The liquid surface at the top of the annular channel is covered by a layer of hydrophobic molecules. The flow is considered as permanent, axisymmetric and creeping. The ratio of the liquid depth to the outer radius is small enough (shallow flow) so that it is possible to develop a matched asymptotic model. In the rotating sub-phase, a core flow is therefore distinguished from the boundary layers along side walls. The modelling includes the possibility to take into account the impact of the radially-inwards molecular packing induced by centrifugation of the underlying bulk. More particularly, radial distribution of surface viscosity is taken into account via the transport equation for surface momentum. In this paper, the model by Mannheimer and Schechter [R.J. Mannheimer, R.S. Schechter, J. Colloid Interface Sci. 32 (2) (1970) 195] can be considered as revisited: a new integral formulation is made evident which enables a fair estimation of the Boussinesq number as well as a simple measurement of a stratified surface viscosity. [Copyright &y& Elsevier]

Abstract: Marangoni migration of a single droplet in an unbounded viscous fluid under the additional effect of variable surface viscosity is studied. The surface tension and the surface viscosity depend on concentration of dissolved species. Cases of the motion induced by the presence of a point source and by a given constant concentration gradient are considered. The dependence of the migration velocity on the governing parameters is computed under quasi-stationary approximation. The effect of weak advective transport is studied making use of singular perturbations in the Peclet number, Pe. It is shown that, when the source is time dependent a Basset-type history term appears in the expansion of the concentration and, as a result, the leading order correction to the flow and to the migration velocity is of . If the source of active substance driving the flow is steady, the effect of convective transport on the migration is weaker. [Copyright &y& Elsevier]

Abstract: Dimensional analysis of the process of steady drainage of liquid from foam has shown that, if surface and inertial effects are neglected, the drainage rate non-dimensionalised as a Stokes-type number may be described by a function of the liquid volume fraction only. In addition it is demonstrated that a simple power law relationship between these two quantities is a good approximation to channel-dominated and node dominated foam drainage models and that the approach enables the prediction of the dependency of drainage upon bubble size. The power law approach requires two adjustable constants which is the minimum required since a priori estimates of both surface shear viscosity and viscous losses in the vertices of the foam are problematical. In addition, a simple dimensionless parameter has been introduced that can modify the power law model to take into account capillary suction so that transients in one-dimensional drainage problems can be modelled. [Copyright &y& Elsevier]

Abstract: Previous analysis of Narsimhan [G. Narsimhan, J. Colloid Interface Sci. 287 (2005) 624–633] for the evaluation of rupture of a nondraining thin film on a solid support due to imposed random mechanical perturbations modeled as a Gaussian white noise has been extended for partially mobile gas–liquid interfaces. The average rupture time of film is evaluated by first passage time analysis (as the mean time for the amplitude of perturbation to become equal to film thickness). The interfacial mobility is accounted for through surface viscosity as well as Marangoni effect. The mean rupture time for partially mobile gas–liquid interface, as characterized by two dimensionless groups, dimensionless surface viscosity and Marangoni number, lies between the two extreme limits for fully mobile and immobile films. The critical wavenumber for minimum rupture time is shown to be insensitive to interfacial mobility. However, the critical dimensionless surface viscosity and critical Marangoni number at which the behavior of thin film deviates from that of fully mobile film and the behavior approaches that of fully immobile film are smaller for higher Hamaker constants, smaller film thickness and smaller surface potentials. [Copyright &y& Elsevier]

Abstract: We report results of drainage in aqueous foams of small bubble size D () prepared with SDS-dodecanol solutions. We have performed free-drainage experiments in which local drainage rates are measured by electrical conductivity and by light scattering techniques. We have investigated the role of the surfactant–cosurfactant mass ratio on the drainage regime. The results confirm that a drainage regime corresponding to a high surface mobility can indeed be found for such small bubbles, and show that an increase in the cosurfactant content can induce a transition to a low surface mobility drainage regime. We show that the transition is not linked to variations of the bulk properties, but rather to variations of the interfacial properties. However, the results show that the added amount of dodecanol to trigger the transition is quite high, evidencing that the relevant control parameter for drainage regimes includes both bubble size and interfacial contributions. [Copyright &y& Elsevier]

We offer a technique that permits the surface viscosity coefficients of LC's to be measured. The author has realized a version of the technique, distinctive feature of which is a usage of membranes having pores of submicron sizes (in the range from 40 µm down to 0.034 μm). We have found out that, at minimum diameter of pores (&nvsim;0.034 µm), the values of viscosity for the azoxy-compound based LC is larger by factor 10–15 compared to that measured with usual (&nvsim;100 µk and more) capillary tubes; the same values for LC 5CB have fond to be larger by factor of 100–350. [ABSTRACT FROM AUTHOR]

Abstract: A theoretical study of the phenomena, occurring in an adsorbed layer, subject to small dilatational perturbations was carried out. Two main processes, provoked by the perturbations (surface reaction and surfactant transport onto the surface) were considered. The reaction was described by means of the reaction coordinate. The derived general rheological equation for insoluble surfactants, gave as limiting cases Voight and Maxwell type equations for fast and slow reactions, respectively. Expressions for all characteristics of the process (surface elasticity, reaction elasticity, reaction relaxation time and dilatational surface viscosity) were obtained. The obtained generalized rheological equation for reactions involving soluble surfactants is a dynamic analog of Gibbs adsorption isotherms for a multi-component system with surface reaction, since similarly to Gibbs equation it relates the surface stress only to surface variables. It gives as limiting cases generalized forms for soluble surfactants of Voight and Maxwell equations. All new rheological equations were analyzed for deformations with constant rate and periodic oscillations and they were applied to three simple surface reactions (monomolecular with one product, dimerization and association). The mass transfer was analyzed initially in the absence of surface reaction. In this system the surface stress is purely elastic, but it was shown that if the adsorption perturbation is small, regardless of the type of surface perturbation and the mechanism of adsorption, the process of mass transport always obeys a Maxwell type rheological equation. For all considered processes surface viscosities were defined, but they were called “apparent”, because they stem from diffusion, rather than from interaction between the surfactant molecules and they depend not only on surface parameters, but also on the geometry of the system. The often used in the literature correlations between the lifetime of emulsions and foams and the imaginary elasticity were analyzed. It was shown that this approach lacks serious scientific foundations and could lead to erroneous conclusions. Finally, the problem for the coupling of the surfactant diffusion with the chemical reaction was analyzed and it was demonstrated on a simple example how it could be tackled. [Copyright &y& Elsevier]

Homeotropic nematic layers of MBBA have been studied by a phase-sensitive flexoelectric spectroscopy method. They have been oriented by films of dilauroyl phosphatidyl choline(DLPC)and cetyl trimethyl-ammonium bromide(CTAB)self-assembled onto the cell glass plates. Using continuum theory, the transmitted light versus excitation frequency spectral shapes have been derived in terms of space variation of the nematic parameters(elastic constant, flexoelectric coefficient, rotational viscosity, and birefringence). These viscoelastic spectra contain information about the surface dissipation of the orientational energy for different aligning films that partially desorb from the surface and dissolve in the nematic, producing a gradient of the surfactant. A new type of flexoelectric effect dependent on the space derivative of flexo-coefficientshas been identified. This effect consists of a bulk flexo-torque source that substantially influences the apparent liquid crystal anchoring. Static and dynamic cases have been analyzed for a steplike surfactant distribution. Resulting spectra have been successfully compared with the experiment, yielding information about the surfactant gradient. These results provide new insights on the interfacial physics of nematic liquid crystals and solid surfaces, where flexoelectricity and desorption play a fundamental role. [ABSTRACT FROM AUTHOR]