Dynamics Of Liquid Spreading Research Articles

Inter-particle liquid spread pertaining to heap leaching using UV fluorescence based image analysis

The visualisation of individual flow paths in a porous media and how they relate to the overall hydrodynamics is crucial to improving our understanding of a number of systems. Unsaturated flow through larger particles (order of millimetres and a few centimetres) in heap leaching systems where both gravity and capillarity play an important role in the structure of the flow paths is complex and still not fully understood. A variety of laboratory methods have been employed for this purpose, with UV light being used in several studies. Flow studies have typically involved millimetre and sub-millimetre scale particles and the flow is typically capillary-dominated. This paper instead concentrates on gravity-dominated flows pertaining to heap leaching and is particularly focused on developing methods for using the UV fluorescence to obtain quantitative measurements of liquid spreading, such as average lateral liquid spread for a point source liquid addition. This study uses a pseudo2-D system with larger ore particles than in previous UV flow studies. The UV fluorescence results were analysed using image analysis to determine the overall liquid spreading and the locations of the flow paths. Both narrowly sized and realistic ore systems were investigated to understand the effect of length scale on flow paths and dynamics of liquid spreading through the gravity-dominated ore beds. It was shown that both ore systems experience distinct flow channelling compared to the more uniform flow profiles observed in capillary-dominated systems. Lateral liquid spreading coefficients were calculated, with the narrowly sized ore system showing higher values, probably due to the larger effective length scale of the inter-particle spaces.

Thin Films in Partial Wetting: Internal Selection of Contact-Line Dynamics.

When a liquid touches a solid surface, it spreads to minimize the system's energy. The classic thin-film model describes the spreading as an interplay between gravity, capillarity, and viscous forces, but it cannot see an end to this process as it does not account for the nonhydrodynamic liquid-solid interactions. While these interactions are important only close to the contact line, where the liquid, solid, and gas meet, they have macroscopic implications: in the partial-wetting regime, a liquid puddle ultimately stops spreading. We show that by incorporating these intermolecular interactions, the free energy of the system at equilibrium can be cast in a Cahn-Hilliard framework with a height-dependent interfacial tension. Using this free energy, we derive a mesoscopic thin-film model that describes the statics and dynamics of liquid spreading in the partial-wetting regime. The height dependence of the interfacial tension introduces a localized apparent slip in the contact-line region and leads to compactly supported spreading states. In our model, the contact-line dynamics emerge naturally as part of the solution and are therefore nonlocally coupled to the bulk flow. Surprisingly, we find that even in the gravity-dominated regime, the dynamic contact angle follows the Cox-Voinov law.

Study of polygonal water bells: inertia-dominated thin-film flows over microtextured surfaces

AbstractMicrotextured surfaces are commonly used to study complex hydrodynamic phenomena such as spreading and splashing of liquid droplets. However, although surface topography is known to modify near-surface flow, there is no theory able to quantitatively predict the dramatic changes in dynamics of liquid spreading and splashing. Here, we investigate experimentally water bells formed on micropatterned surfaces in order to characterize the hydrodynamics of inertia-dominated flows through regular porous layers. Water bells are self-suspended catenary-shaped liquid films created when a jet impinges on a horizontal disc called an impactor. We show that the presence of micrometre-sized posts regularly arranged on the impactor results in a decrease of the water bell radius and the loss of axisymmetry as open water bells adopt polygonal shapes. We introduce a simple model that captures the main features of the inertia-dominated flow and reveals the role of the hydrodynamic interactions between neighbouring posts. In addition to their applications for tunable jet atomization, these polygonal sheets provide a paradigmatic system for understanding inertia-dominated flow in porous media.

Smart photochromic gratings with switchable wettability realized by green-light interferometry

We demonstrate the enhancement of the wetting properties of smart photochromic surfaces by a specifically developed, gentle interferometric patterning employing green light. We realized photochromic gratings with 2.5–10.0μm period by a blend consisting of a green-curable matrix and spiropyran molecules. The structured surfaces exhibit photocontrolled and reversible wettability, and enhanced hydrophilicity with respect to the native substrates. The dynamics of liquid spreading onto the gratings was also investigated, and the wetting behavior analyzed according to Wenzel’s [Ind. Eng. Chem. 28, 988 (1936)] model for rough surfaces. These results indicate switchable gratings as promising functional components for microfluidics and modulation, and green-light interferometry as a reliable lithographic method for patterning organics without degradation or photochemical reactions.

Spreading dynamics of liquids and surfactant solutions on partially wettable hydrophobic substrates

The drop spreading of water and aqueous solutions of ethanol and nonionic surfactant on hydrophobic substrates (alkylsilane treated glass) have been investigated. For the low viscous liquids and solutions, the spreading on the surface of hydrophobic glass rod was also studied and compared to the drop spreading experiment. In both experiments, care was taken to ensure a minimum impact of inertial forces. The results for the aqueous systems show rapid initial spreading processes that abruptly halts after less than 30 ms, as the interfacial tension forces are balanced. In the case of surfactants solutions, this is followed by slower adsorption driven drift towards equilibrium conditions. During the initial spreading phase, the wetting front exhibits ∼ t 1/2 spreading law. Two more viscous liquids, ethylene glycol and glycerol, were also examined and found to show a weaker time-dependence in the whole spreading regime. An ∼ t 1/10 scaling of the drop radius versus time was for these liquids observed in the asymptotic long-time regime. For the surfactant solution, a slow relaxation towards equilibrium was observed following the initial fast spreading phase. The rate-limiting process in this regimes was in the drop spreading experiment found to be surfactant adsorption from the bulk to the expanding liquid–vapour interface, whereas surface diffusion at the liquid–vapour interface appeared rate-determining in the rod experiment. The reason for this is the differences in aspect ratio between relative expansion of the liquid–vapour and solid–liquid interfaces during spreading in the two experiments. In the study of surfactant solution spreading, the importance of surface relaxation prior to contact of the solution and the solid was also pointed out.

Dynamics of Liquid Spreading on Solid Surfaces

Using simple scaling arguments and a precursor film model, we show that the appropriate macroscopic contact angle θ during the slow spreading of a completely or partially wetting liquid under conditions of viscous flow and small slopes should be described by tan θ = [tan3 θe − 9 log ηCa]1/3 where θe is the static contact angle, Ca is the capillary number, and η is a scaled Hamaker constant. Using this simple relation as a boundary condition, we are able to quantitatively model, without any empirical parameter, the spreading dynamics of several classical spreading phenomena (capillary rise, sessile, and pendant drop spreading) by simply equating the slope of the leading order static bulk region to our dynamic contact angle boundary condition without performing a matched asymptotic analysis for each case independently as is usually done in the literature.

On the dynamics of liquid spreading on solid surfaces

Our main objective is to identify a boundary-value problem capable of describing the dynamics of fluids having moving contact lines. A number of models have been developed over the past decade and a half for describing the dynamics of just such fluid systems. We begin by discussing the deficiencies of the methods used in some of these investigations to evaluate the parameters introduced by their models. In this study we are concerned exclusively with the formulation of a boundary-value problem which can describe the dynamics of the fluids excluding that lying instantaneously in the immediate vicinity of the moving contact line. From this perspective, many of the approaches referred to above are equivalent, that is to say they give rise to velocity fields with the same asymptotic structure near the moving contact line. Part of our objecive is to show that this asymptotic structure has only one parameter. A substantial portion of our investigation is devoted to determining whether or not the velocity field in a particular experiment has this asymptotic structure, and to measuring the value of the parameter.More specifically, we use the shape of the fluid interface in the vicinity of the moving contact line to identify the asymptotic structure of the dynamics of the fluid. Experiments are performed in which silicone oil displaces air through a gap formed between two parallel narrowly-spaced glass microscope slides sealed along two opposing sides. Since we were unable to make direct measurements of the shape of the fluid interface close to the moving contact line, an indirect procedure has been devised for determining its shape from measurements of the apex height of the meniscus. We find that the deduced fluid interface shape compares well with the asymptotic form identified in the studies referred to above; however, systematic deviations do arise. The origin of these deviations is unclear. They could be attributed to systematic experimental error, or, to the fact that our analysis (valid only for small values of the capillary number) is inadequate at the conditions of our experiments.