Session G6: Surface Tension Effects

Acoustically-Enhanced Direct Contact Vapor Bubble Condensation

Rate-limited, direct contact vapor condensation of vapor bubbles that are formed by direct steam injection through a nozzle in a quiescent subcooled liquid bath is accelerated using ultrasonic (MHz-range) actuation. A submerged, low power actuator produces an acoustic beam whose radiation pressure deforms the liquid-vapor interface, leading to the formation of a liquid spear that penetrates the vapor bubble to form a vapor torus with a significantly larger surface area and condensation rate. Ultrasonic focusing along the spear leads to the ejection of small, subcooled droplets through the vapor volume that impact the vapor-liquid interface and further enhance the condensation. High-speed Schlieren imaging of the formation and collapse of the vapor bubbles in the absence and presence of actuation shows that the impulse associated with the collapse of the toroidal volume leads to the formation of a turbulent vortex ring in the liquid phase. Liquid motions near the condensing vapor volume are investigated in the absence and presence of acoustic actuation using high-magnification PIV and show the evolution of a liquid jet through the center of the condensing toroidal volume and the formation and advection of vortex ring structures whose impulse appear to increase with temperature difference between the liquid and vapor phases. High-speed image processing is used to assess the effect of the actuation on the temporal and spatial variations in the characteristic scales and condensation rates of the vapor bubbles.   

Numerical simulation of compressible bubble motion with phase change

Numerical simulation is performed for bubble motion under a pressure wave condition, which receives increasing attention in medical therapy and targeted drug delivery. The level-set method for incompressible two-phase flows is extended to include the effect of liquid and vapor compressibility as well as the effect of phase change by incorporating the ghost fluid method to efficiently implement the matching conditions of velocity, stress and temperature at the interface. The semi-implicit pressure correction formulation is implemented into the level-set method to avoid the serious time step restriction in weakly compressible flows. The numerical results for 1-D compressible flows show good agreement with the analytical solutions. The computation of bubble motion under a periodic pressure wave condition demonstrates that the bubble in a compressible liquid significantly amplify the incoming wave at the resonance condition and builds up the liquid pressure to a very high level. The computation of vapor bubble motion in a subcooled liquid shows that the condensation heat transfer to the colder liquid causes very rapid bubble collapse and strong pressure wave propagation. The effects of pressure amplitude and ambient temperature on the bubble motion are investigated.   

Cavitation at the air/water interface induced by CO$_{\mathrm{2}}$ laser: formation, dynamics and mechanism

We report CO$_{\mathrm{2}}$-laser-induced cavitation at the interface between air and water, since strong photo-thermal effect of water occurs at the infrared wavelength. Using high-speed camera, we record explosive evaporation and the evolution of cavitation at the interface. By analyzing the growth dynamics of cavitation at various experimental conditions, we identify two stages of its growth associated with different mechanisms correspondingly. One stage is an initial faster expanding process driven by the influx of dissolved gas expelled from the surrounding water due to laser heating; and the other stage is a subsequent slower isobaric expanding process related with liquid properties. More quantitatively, we find that the evolution of cavitation at first stage is characterized by scaling law with an exponent of 1/3 for its diameter as a function of time, while its evolution at the second stage can be well described by Rayleigh-Plesset theory. This study of interfacial cavitation due to photo-thermal effect might have implications for solar-steam technology and infrared-laser surgery as well.   

Liquid rims collisions and the formation of fines

As an elementary mechanism for the formation of drops from liquid sheets, we investigate the collision of liquid cylinders. This results from the opening of two nearby holes on a liquid film, growing at a constant speed while collecting liquid into two rims, eventually colliding with each other. In this surface tension driven phenomenon, a unique Weber number $We = \rho (2V)^2 2a/\sigma$ controls a variety of behaviors ($\rho, \sigma$ are the liquid density and surface tension, and $2V$ the relative velocity of the impinging rims, each of individual radius $a$). At low $We$, the rims merge through an inelastic, dissipative collision which produces a corrugated ligament, finally breaking into drops of size scaling like $a$, on average. Above a critical $We_c \approx 60$, the collision leads to a splash, with the formation of a thin transverse liquid sheet. We will describe the expansion-retraction dynamics of this secondary sheet and its destabilization, responsible for the production of a mist of finer droplets. These alter sensibly the mean, and overall drops size distribution, thus weighted by a substantial fraction of so-called fines.   

Simulation on the Effects of Surfactants and Observed Thermocapillary Motion for Laser Melting Physics

A newly developed, robust, high-order in space and time, Newton-Krylov based reconstructed discontinuous Galerkin (rDG) method is used to model and analyze thermocapillary convection in melt pools. The application of interest is selective laser melting (SLM) which is an Additive Manufacturing (AM, 3D metal laser printing) process. These surface tension driven flows are influenced by temperature gradients and surfactants (impurities), and are known as the Marangoni flow. They have been experimentally observed in melt pools for welding applications, and are thought to influence the microstructure of the re-solidified material. We study the effects of the laser source configuration (power, beam size and scanning speed), as well as surfactant concentrations. Results indicate that the surfactant concentration influences the critical temperature, which governs the direction of the surface thermocapillary traction. When the surface tension traction changes sign, very complex flow patterns emerge, inducing hydrodynamic instability under certain conditions. These in turn would affect the melt pool size (depth) and shape, influencing the resulting microstructure, properties, and performance of a finished product part produced using 3D metal laser printing technologies.   

Indenting a Thin Floating Film: Force and First-fold Formation

When a thin elastic sheet is gently pushed into a liquid bath, a pattern of radial wrinkles is generated where the film is locally compressed. Despite the simplicity of this setting, basic questions remain about the mechanics and morphology of indented thin films. Recent work shows that traditional post-buckling analysis must be supplanted with an analysis where wrinkles completely relax compressive stresses. Support for this “far-from-threshold” theory has been built on measurements of wrinkle extent and wavelength, but direct force measurements have been absent. Here we measure the force response of floating ultrathin ($\sim$100 nm) polystyrene films in indentation experiments. Our measurements are in good agreement with recent predictions for two regimes of poking: Early on force depends on film properties (thickness and Young`s modulus) and later is independent of film properties, simply transferring forces from the substrate (gravity and surface tension) to the poker [1]. At larger indentations compression localizes into a single fold [2]. We present scaling arguments and experiments that show the existing model of this transition must be modified. [1] Vella et al., Phys. Rev. Lett. 114, 014301 (2015). [2] Holmes and Crosby, Phys. Rev. Lett. 105, 038303 (2010).   

Adsorption Of Surfactants At the Water-Oil Interface By Short-Time Diffusion

Surface tension is an important parameter for different industrial processes. The addition of surfactants can modify the interfacial tension between two fluids. As the surfactant molecules reach and are adsorbed at a fluid interface, the surface tension or interfacial tension is reduced until the interface is saturated. Dynamic Interfacial tension measurements were carried out using an optical tensiometer by the Pendant Drop technique at a room temperature of 25 \textdegree C for a period of 250 sec. A drop of surfactant solution was deposited and allowed to diffuse into a water--oil interface, and then the adsorption rate at the interface was calculated. Sodium Dodecyl Sulfate (SDS) was used as the surfactant, hexane and dodecane were tested as the oil phase. A linear decay in the interfacial tension was observed for the lower initial concentrations of the order of 0.0001 to 0.01 mM, and an exponential decay was observed for initial concentrations of the order of 0.1 to 1 mM.   

Pairwise Hydrodynamic Interaction of Particles Straddling Flat and Curved Fluid Interfaces

Numerical solutions are obtained for the hydrodynamic interaction, in the creeping flow regime, for two identical particles floating at an air/liquid interface as a function of the inter-particle separation distance and their immersion depth into the liquid phase. Drag coefficients are computed for all modes (components of the resistance matrix) for translational motion along and perpendicular to the inter-particle line of centers for particles confined to the interface. The largest interaction when the particles mutually approach one another. For a flat free interface, the interaction of mutual approach for hydrophilic particles was found to be larger than that in the bulk, while reverse effect was observed for hydrophobic particles. The coefficients are used to compute the trajectories for the mutual approach of particle pairs subject to attractive capillary forces. We then imposed a curvature to the interface and re-calculate the drag coefficients as a function of the immersion depth, separation distance and quotient of the particle size to interface radius of curvature.