Surface tension gradient driven spreading of trisiloxane surfactant solution on hydrophobic solid

We investigate effects of surface-tension gradients on the performance of a micro-grooved heat pipe in this work. The surface-tension gradient force is accounted for in the present model, and expressions for radius of curvature, liquid pressure, liquid velocity, and maximum heat throughput are found analytically using a regular perturbation technique. With a favorable surface-tension gradient, the liquid pressure drop across the heat pipe can be decreased by ∼90%, and the maximum heat throughput can be increased by ∼20%. In contrast, using an unfavorable surface-tension gradient, the liquid pressure drop increases by ∼150%, and the maximum heat throughput decreases by ∼15%. For the same values of the favorable and unfavorable surface-tension gradients, the unfavorable effect is more pronounced than the favorable one. The effects of the surface-tension gradients are found to be increasing with the corner angle of a polygonal heat pipe. Adverse effects of the surface-tension gradient could be due to the variations in the liquid temperature and/or surfactant concentration. Nevertheless, a favorable situation where the surface-tension gradient can facilitate the liquid flow in a heat pipe can also be obtained using a suitable surfactant, surface charge, etc., and then the performance of a micro heat pipe can be improved.

A biharmonic stream function equation is solved analytically in axisymmetric toroidal coordinates for Stokes flow in a sessile droplet with a surface tension gradient and diffusive evaporation on the surface. Four different types of streamlines are defined and utilized to predict particle deposition either at the center or at the contact line of the droplet. The impact of UV light radius, droplet's contact angle, and humidity level on the flow pattern is investigated. When the relative humidity is high, the formation of Moffatt eddies is observed. However, the eddy size and the number of eddies decrease with an increase in UV light radius or droplet contact angle. As humidity decreases, the Moffatt eddies are restricted to a specific region, decrease in number and size, and ultimately disappear. Under lower humidity, the Marangoni flow near the contact line is dominated by capillary action. The toroidal streamlines are broken from outside and turned into transporting streamline to deposit more particles on the contact line. The region near the droplet center, where the surface tension gradient is significant, is always dominated by toroidal streamlines.

Colloidal fluids are used in a variety of technological contexts. For example, their spreadingand adhesion behavior on solid surfaces can yield materials with desirable structural and optical properties.The well-established concepts of spreading and adhesion behavior of simple liquids do not apply to colloidalfluids containing nanometer-sized particles, surfactant micelles, proteins, polymers, vesicles, microemulsions,and solvents. This paper reviews recent progress in the spreading of colloidal/nano-fluids over solid surfaceswith emphasis on two applications: the spreading of aqueous trisiloxane surfactant solutions (i.e., superspreaders)on hydrophobic solid surfaces driven by the surface tension gradient, and the spreading of thin colloidalfilms containing nanoparticles on hydrophilic surfaces driven by the structural disjoining pressure gradient(i.e., film tension gradient). These two mechanistic paradigms of dynamic spreading of colloidal fluidson solids are elucidated with experimental observations and mathematical modeling.

Achieving control over the motion of dissolved particles in liquid metals is of importance for the meticulous realization of hierarchical particle assemblies in a variety of nanofabrication processes. Brownian forces can impede the motion of such particles, impacting the degree of perfection that can be realized in assembled structures. Here, we show that light-induced Marangoni flow in liquid metals (i.e., liquid-gallium) with Laguerre-Gaussian (LGpl) lasers as heating sources is an effective approach to overcome Brownian forces on particles, giving rise to predictable assemblies with a high degree of order. We show that by carefully engineering surface tension gradients in liquid-gallium using non-Gaussian LGpl lasers, the Marangoni and convective flow that develops in the fluid drives the trajectory of randomly dispersed particles to assemble into 100 μm wide ring-shaped particle assemblies. Careful control over the parameters of the LGpl laser (i.e., laser mode, spot size, and intensity of the electric field) can tune the temperature and fluid dynamics of the liquid-gallium as well as the balance of forces on the particle. This in turn can tune the structure of the ring-shaped particle assembly with a high degree of fidelity. The use of light to control the motion of particles in liquid metals represents a tunable and rapidly reconfigurable approach to spatially design surface tension gradients in fluids for more complex assembly of particles and small-scale solutes. This work can be extended to a variety of liquid metals, complementary to what has been realized in particle assembly out of ferrofluids using magnetic fields.

We report on a new hydrodynamic instability which occurs at the spreading edge of a thin wetting film. A drop of aqueous surfactant solution placed on a glass surface moistened with a thin layer of water spreads by propagating fingers, whose velocity and shape depend on the thickness of the ambient water layer and on the surfactant concentration. The two fluids are miscible and show negligible viscosity difference, ruling out a Saffman-Taylor instability. We propose that the Marangoni effect, which is fluid flow induced by gradients in surface tension, drives the instability.

Soil flushing by surfactant solution: Pilot-scale demonstration of complete technology

Association properties of two silicone surfactants based on poly(dimethylsiloxane)-graft-polyethers in aqueous solutions of 2-butoxyethanol, poly(ethylene glycol), and glucose were determined using various techniques such as surface tension, small-angle neutron scattering, and viscosity. Dilute solution phase diagrams were also constructed, and cloud points were measured for different concentrations of both the surfactants in additive aqueous solutions. The thermodynamic parameters for the micellization were obtained from the temperature-dependent data on critical micelle concentration. The influence of given additive on the micellization of silicone surfactants was monitored from the changes in the free energy of micellization values for the surfactant solutions in water and water + additives. The cloud points and the critical micelle concentration values for the surfactant solutions were found to be decreased in the presence of the selected three additives. The analysis of the changes in the relative permittivities and partial molar volumes for the surfactant solutions in the presence of water and water + additives suggests that the solvent environment around the surfactant solute molecules is different in mixed solvent systems vis-a-vis water and the micellization and surface activity of silicone surfactants were dictated predominantly by the preferential hydration of cosolute additives. The analysis of small angle neutron scattering curves for the surfactant aqueous solutions in the presence of additives showed that the micelles formed have oblate ellipsoidal shape at 30 °C with, however, increased characteristic axial ratios than those in pure water. The increase in the concentration of additives has also increased the size of the micelles. The increase in temperatures corresponding to the values close to the turbid boundaries in the phase diagrams caused a transition from the oblate to the disklike micellar shape. The changes in the hydration values of the micellar associates were monitored from the dilute solution viscosity measurements. The dehydration of the micelles in the presence of additives at 30 °C and also at elevated temperatures occur from the interior to the fringe of the core−outer shell parts and to the outer shell successively.

Bubble surface mobility regimes in mixed aqueous solutions of short chain alcohols and fluorosurfactant.

Effect of nonionic surfactant on wetting behavior of an evaporating drop under a reduced pressure environment

Marangoni flow is the motion induced by a surface tension gradient along a fluid–fluid interface. In this study, we report a Marangoni flow generated when a bath of surfactant contacts a pre-wetted film of deionized water on a vertical substrate. The thickness profile of the pre-wetted film is set by gravitational drainage and so varies with the drainage time. The surface tension is lower in the bath due to the surfactant, and thus a liquid film climbs upwards along the vertical substrate due to the surface tension difference. Particle tracking velocimetry is performed to measure the dynamics in the film, where the mean fluid velocity reverses direction as the draining film encounters the front of the climbing film. The effect of the surfactant concentration and the pre-wetted film thickness on the film climbing is then studied. High-speed interferometry is used to measure the front position of the climbing film and the film thickness profile. As a result, higher surfactant concentration induces a faster and thicker climbing film. Also, for high surfactant concentrations, where Marangoni driving dominates, increasing the film thickness increases the rise speed of the climbing front, since viscous resistance is less important. In contrast, for low surfactant concentrations, where Marangoni driving balances gravitational drainage, increasing the film thickness decreases the rise speed of the climbing front while enhancing gravitational drainage. We rationalize these observations by utilizing a dimensionless parameter that compares the magnitudes of the Marangoni stress and gravitational drainage. A model is established to analyse the climbing front, either in the Marangoni-driving-dominated region or in the Marangoni-balanced drainage region. Our work highlights the effects of the gravitational drainage on the Marangoni flow, both by setting the thickness of a pre-wetted film and by resisting the film climbing.

The equations that govern the dynamics of the liquid–vapor interface and contact line of a sessile drop in the spreading and evaporation–condensation regimes are derived. During spreading, the liquid–vapor interface and contact line convect with the liquid and are therefore material. In contrast, when evaporation or condensation occurs, the liquid–vapor interface and contact line migrate relative to the liquid and are therefore nonmaterial. For spreading, the evolution equations consist of kinematical constraints on the normal velocities of the liquid–vapor interface and contact line along with the normal and tangential components of the constitutively augmented standard force balances on the liquid–vapor interface and along the contact line. The tangential components of the standard force balances on the liquid–vapor interface and at the contact line, being solely dissipative, are automatically satisfied at equilibrium. The normal component of the standard force balance on the liquid–vapor interface reduces to the Young–Laplace equation, whereas its counterpart along the contact line simplifies to the generalization, accounting for line energy, of the Young equation mentioned by Gibbs. In the presence of evaporation or condensation, the kinematical constraints are no longer valid. Their absence is compensated for by the normal components of the configurational force balances on the liquid–vapor interface and along the contact line. Hence, away from equilibrium and in the presence of dissipation, a complete description of the liquid–vapor interface and contact line of a volatile drop involves the normal and tangential components of the standard force balances along with the normal components of the configurational force balances. At equilibrium, the normal component of the configurational force balance on the liquid–vapor interface simplifies to the condition for chemical equilibrium requiring the continuity of the chemical potential, whereas its counterpart along the contact line, being entirely dissipative, holds trivially. Comparison with recently proposed kinetic laws for the liquid–vapor interface and contact line of a drop is provided.

This work compares the spreading and relaxation rates of albumin and fibrinogen, inferred from single-component and competitive adsorption kinetic experiments, on model surfaces of varying hydrophobicity. Kinetics from the single-component studies revealed a constant spreading rate, where the adsorbed protein footprint grew linearly in time for at least 15 min. This spreading rate increased with substrate hydrophobicity (ranging from 0.02 to 0.16 nm2/molecule/s for albumin and from 0.04 to 0.26 nm2/molecule/s for fibrinogen), resulting in a larger extent of footprint growth and a lower ultimate coverage on hydrophobic surfaces when compared with hydrophilic surfaces at the same adsorption conditions. Competitive adsorption studies were in qualitative agreement with the single-component experiments but were able to probe longer spreading time scales. Although spreading appeared to occur initially at a constant rate in the competitive experiments, after 2 h the spreading rate had slowed dramatically and the...

Problems encountered in applying the marginal regeneration concept introduced by Mysels for describing drainage of free liquid films with mobile surfaces include the understanding of the dependence of the phenomenon on direction. This can be explained by the surface tension gradient in the film: new film elements entering from the border stretch especially in the vertical direction. Therefore the film/border transition at the top of a film drawn in a frame does not show turbulence caused by marginal regeneration. At the bottom it does show marginal regeneration phenomena but these are there not very active in promoting drainage because new film elements stretch in a direction perpendicular to the border/film contact line. At the vertical film/border transitions the new film elements stretch along the contact line and by so doing reinforce outflow from adjacent regions. This makes these border/film contact regions especially active with regard to film drainage.

We study the effect of Marangoni flow on a dynamic contact line formed by a propagating reaction front and a liquid-air interface. The self-sustained iodate-arsenous acid reaction maintains the production of the weakly surface active iodine leading to an unbalanced surface force along the tip of the reaction front. The experiments, performed in microgravity to exclude the contribution of buoyancy, reveal that the fluid flow generated by the surface tension gradient is localized to the contact line. The penetration depth of the surface stress is measured as 1-2 mm; therefore, with greater fluid height the liquid advancement on the upper surface does not lead to enhanced mixing in the bulk. Because the propagation velocity of the reactive interface remains at that of reaction-diffusion, the leading edge consists of two straight lines; a tilted segment connects the contact line on the surface with the vertical segment on bottom. Modeling calculations of the reaction-diffusion-advection system in three dimensions reconstruct the experimental observations and along with the experiments validate a model based on geometric spreading. According to the calculated flow field, the direction of significant fluid flow follows the concentration gradients and hence coincides with the propagation of the reaction front, allowing only negligible transverse flow in the upper fluid layer.

Unique role of ionic liquid [bmin][BF 4] during curcumin–surfactant association and micellization of cationic, anionic and non-ionic surfactant solutions