Session HP: Multiphase Flows IV

The Effect of Surface Wettability on Viscous Film Deposition

The viscous deposition of a liquid film on the inside of a capillary has been experimentally investigated and the relationship between the film thickness and surface wettability was examined. With distilled water as a working fluid tests were run in a 500 microns diameter glass tube with less than 30 degrees and 105 degrees contact angle. The thickness \textbf{h} of the deposited film was then estimated from the liquid mass flow rate exiting the capillary and the gas-liquid interface (meniscus) velocity, and compared with Taylor's data and with modified Bretherton's correlation as a function of the Capillary number. In a different set of experiments direct film thickness measurements were obtained by matching the refractive index of the capillary with that of the investigated fluid. The tube was also placed in an index-matched view box to minimize distortion and allow for accurate evaluation of the film thickness. The results were checked against data resulting from the aforementioned procedure. The thickness measurements as well as the meniscus velocity were determined with the aid of a Photron high speed camera with 10000 frames per second sampling capability coupled with a Nikon TE-2000 inverted microscope.   

Wipe Coating from a Partially Saturated Porous Material

In the last few years, there has been a dramatic increase in the number of consumer products where a porous media is used to deliver a fluid with a desired functionality. A partially saturated porous material wiped against a hard surface leaves a coating of fluid behind and may be seen as a coating process. Nobody addressed this problem before in the open literature and there is no model describing its dynamics. From this point of view, it is a new process and we will call it ``wipe coating.'' We present a model which gives the film thickness dependence on velocity, viscosity, surface tension and transport properties of the non- deformable porous material, when it wipes on a hard smooth surface, it is partially saturated with a Newtonian liquid and the variation in saturation is negligible. The model is validated through experimental measurements run at different saturations, velocities and permeabilities. The agreement is very good, with the adjustment of only one free parameter which includes the partial permeability of the medium. Complimentary experiments confirmed the value obtained for the fitting parameter.   

Buoyancy driven interpenetration of immiscible fluids of different densities in a tilted tube

The buoyancy driven interpenetration of two immiscible fluids of different densities and equal viscosities (salt water solutions and silicon oil) initially separated by a gate valve has been studied in a long tilted tube of small diameter ($20 \mathrm{mm}$). For increasing tilt angles $\theta$ from vertical, one observes first a turbulent flow where each phase becomes fractionated into many bubbles. Then, each fluid penetrates into the other, taking the shape of a large bubble of length increasing with time. The rear end of the bubble may either remain ``attached'' to the gate valve or move at a velocity lower than the front end. At tilt angles close to horizontal, each bubble of displacing fluid often becomes divided into a sequence of large bubbles or, alternatively, is replaced by a thin layer of fast moving fluid. The velocities of front and rear ends have been studied as a function of the experimental parameters and compared to usual characteristic velocities for large bubbles rising into another fluid. The influence of the density contrast between the fluids and of the initial conditions (tube cleaned before the experiment or prewetted by a previous displacement) are discussed.   

Dependence of local void fraction distribution on turbulent structure of upward bubbly flow in vertical channel

Turbulent structure of upward dilute bubbly flow with 1 mm bubbles in a vertical channel is investigated experimentally. Small amount of surfactant is added to water to avoid bubble coalescence and to control local void fraction distribution. Liquid phase velocity is measured using two-dimensional Laser Doppler Velocimetry. In 1-Pentanol solution of 20 ppm, bubbles have half-slip surface and migrate strongly toward the channel wall due to the shear-induced lift force which leads to wall-peaked distribution of local void fraction. On the other hand, in Triton X-100 solution of 2 ppm, bubbles become fully-contaminated and do not migrate toward the wall or the channel centre due to near-zero lift force, causing uniform distribution of local void fraction in the wall-normal direction. Once bubbles accumulate near the wall, transport of turbulent energy produced by the wall shear towards the channel centre is blocked. Then turbulence induced by the bubble motion becomes dominant in a wide core region (so-called pseudo turbulence). By contrast, in the case of uniform distribution of bubbles, a mechanism of a turbulent energy transport which is the same as that of a single-phase turbulence still exists and furthermore the bubble-induced turbulence is added on it.   

Vapor bubble dynamics in confined geometries

Vapor bubble dynamics in two different confined geometries has been studied experimentally and theoretically. Individual vapor bubbles were created by laser pulses focused in water-filled cylindrical glass tubes (1D) and in between two parallel plates (2D). The control parameters are the energy input and the confinement of the bubble, i.e. the diameter of the microtube or the separation between the two plates, respectively. It is found to be independent of the contact angle. Traditional models of vapor bubble growth, which only consider inertia and viscous friction, fail to describe the observed vapor bubble dynamics. However, by in addition considering the heat transfer between the liquid and the vapor bubble, we can quantitatively account for the experimental data, both during growth and collapse of the bubble.   

Numerical studies of droplet impacts on homogeneous and heterogeneous substrates

We present numerical results of droplet impacts on heterogeneous substrate as well as homogeneous substrates. The numerical method is based on the level set method, the THINC/WLIC method (a VOF type method), the CIP-CSL method and the CSF model. We compare the numerical results with experiments. The numerical results show good agreements with the experimental data. We compare numerical results obtained by several kinds of contact angle models.   

Pairing and Collective Dynamics of Particles and Deformable Drops in Parallel-Wall Channels

Fluids used in microfluidic applications often consist of multiple phases, i.e. polymer blends, blood and biological mixtures. One application is the generation of a regular array by the use of T-junctions and flow-focusing devices. In this work, we focus on the pairing and collective dynamics of these trains, and in particular on the influence of the deformability of the dispersed phase, by comparing trains of solid particles and trains of drops. Numerical methods employed are boundary integrals for drops and Stokesian-dynamics techniques for solid particles. We show that isolated pairs of drops undergo pairing, while isolated pairs of rigid spheres do not cluster. By contrast, confined linear arrays of particles and drops always undergo pairing regardless of deformability. Depending on the deformability and the initial separation between the drops, the initial dynamics of the pairing behavior can be quite complex. At prolonged time scales, all drop pairs reach the same velocity, while particle pairs migrate at a different velocity due to different intra-particle distances. In addition, the response of linear arrays to particle displacements shows a qualitative dependence on deformability. For example, drops are self-centering between the bounding walls and therefore linear arrays of drops are more stable to displacements normal to the walls. Complex collective behavior is also observed for linear arrays with particle and drop displacements parallel to the walls.   

Micromorphic Theory of Bubbly Fluid Mixtures

We use a continuum theory for multiphase immiscible mixtures with inner structure. Based on micromorphic theory, the average balance equations for the different phases, as well as for the mixture, result from a systematic averaging procedure. In addition to equations for mass, momentum, energy and entropy, the balance equations also include equations for microinertia and microspin tensors. These equations, together with appropriate constitutive equations consistent with the entropy inequality, enable the modeling of immiscible multiphase materials where internal parameters are important. Here, we apply the results to a two-phase simple microstretch (expansion or contraction) bubbly fluid mixture. We show that the equations for microspin and microinertia, under a number of simplifying assumptions, combine to yield a general form of the Rayleigh-Plesset equation.   

Controlled deposition of drops on edible film using an AC-electric field

We present a drop-based technique for the deposition of drug dosages onto an edible film. Unlike the common approach to utilize DC electric fields and metal nozzles, we use a nozzle made of an electrically insulating material and apply an AC electric field to form drops. Experiments were conducted on polyethylene glycol (PEG)-based solutions over a broad range of the applied frequency and the applied peak-to-peak voltage. Presented results demonstrate the various drop formation regimes observed as a function of the field strength and frequency. We discuss the mechanism of the drop formation in an insulating nozzle caused by the application of an AC field.