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Wetting, which addresses either spontaneous or forced spreading of liquids on a solid surface, is a ubiquitous phenomenon in nature and can be observed by us on a daily basis, e.g., rain drops falling on a windshield and lubricants protecting our corneas. The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. Generally, wetting is governed by the dynamic and static behaviors of the three-phase contact line. Therefore, a deep insight into the dynamics and statics of three-phase contact line at nanoscale is necessary for the technological advancement in nanotechnology and nanoscience. This dissertation aims to understand the dynamic wetting under a molecular kinetic framework and resolve the reconfiguration of liquid molecules at the molecular region of contact line. Water spreading on polytetrafluoroethylene surfaces is selected as a classical example to study the dynamic behaviors of three-phase contact line. To accommodate the moving contact line paradox, the excess free energy is considered to be dissipated in the form of molecular dissipation. As-formed contact line friction/dissipation coefficient is calculated for water interacting with PTFE surfaces with varying structures and is found to be on the same order of magnitude with dynamic viscosity. From an ab initio perspective, contact line friction is decomposed into contributions from solid-liquid retarding and viscous damping. A mathematical model is established to generalize the overall friction between a droplet and a solid surface, which is able to clarify the static-to-kinetic transition of solid-liquid friction without introducing contact angle hysteresis. Moreover, drag reduction on lotus-leaf-like surface is accounted for as well. For the first time, the concept of contact line friction is used in the rational design of a superhydrophobic condenser surface for continuous dropwise condensation. We focus on the transport and reconfiguration of liquid molecules confined by a solid wall to shed light on the morphology of the molecular region of a three-phase contact line. A governing equation, which originates from the free energy analysis of a nonuniform monocomponent system, is derived to describe the patterned oscillations of liquid density. By comparing to the Reynolds transport theorem, we find that the oscillatory profiles of interfacial liquids are indeed governed in a combined manner by self-diffusion, surface-induced convection and shifted glass transition. Particularly for interfacial water, the solid confining effects give rise to a bifurcating configuration of hydrogen bonds. Such unique configuration consists of repetitive layer-by-layer water sheets with intra-layer hydrogen bonds and inter-layer defects. Molecular dynamics simulations on the interfacial configuration of water on solid surfaces reveal a quadratic dependence of adhesion on solid-liquid affinity, which bridges the gap between macroscopic interfacial properties and microscopic parameters. / Doctor of Philosophy / The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. The conventional hydrodynamic analysis with no slip boundary condition predicts a diverging shear stress at the contact line as well as an unbounded shear force exerted on the solid surface. To accommodate this paradox, different mechanism and models have been proposed to clarify the slip between a moving contact line and a solid surface. Although almost all models yield reasonable agreement with experimental observations or numerical simulations, it is still difficult to pick up a specific model using only macroscopic properties and experiment-accessible quantities, because the energy dissipation mechanism during dynamic wetting is not identified and the contact line deforms over different length scales. In this dissertation, we ascribe the energy dissipation in dynamic wetting to contact line friction/dissipation under the framework of molecular kinetic theory, as it is assumed that the contact line is constantly oscillating around its equilibrium position. By decomposing contact line friction into two contributions: solid-liquid retarding and viscous damping, we are able to derive a universal model for the contact line friction. This model predicts a decaying solid-liquid friction on superhydrophobic surfaces, corresponding to the lotus effect. In the meantime, this model is able to clarify the recently-discovered static-to-kinetic transition of frictional force between a sessile drop and a solid surface. Later, we used the concept of contact line friction in the droplet growth process in dropwise condensation so as to promote the rational design of superhydrophobic condenser surfaces for sustainable dropwise condensation. As the morphology of a contact line is dependent on the length scale of interest, we focus on the molecular region of contact line. We study the transport and structural change of liquid molecules that are several molecular layers away from the solid surface. It is found that liquid molecules in this region experience patterned density oscillations, which cannot be simply attributed to the random deviations from continuum limit. By combining free energy analysis and Reynolds transport theorem, it is demonstrated that the omnipresent density oscillations arise from the collective effects of self-diffusion, surface-induced convection and shifted glass transition. For liquid water, we propose a bifurcating hydrogen bonding network in contrast to its tetrahedron configuration in bulk water.

The effect of fabric structure and yarn-to-yarn liquid migration on the overall liquid transport behavior of fabrics is investigated in this research. Sorption of liquid from an unlimited reservoir as well as sorption of a limited quantity of liquid by fabrics representing different structural parameters is studied in detail. Sorption of a limited quantity of liquid is studied by performing drop spreading experiments on fabrics. The spreading and wicking of micron sized drops which are deposited on textile fabrics during ink jet printing is also studied. How the fabric structure related variables influence the spreading of ink drops and how exactly spreading influences printing quality is investigated in this research. Results showed that the wicking in fabrics is determined by the wicking rates of the yarns, thread spacing and more importantly by the rate at which liquid migrates from longitudinal to transverse threads and again from transverse threads back to longitudinal threads. Drop spreading rates were also determined by fabric structure. In general, compact and thinner fabrics showed highest drop spreading rates. Drop spreading rates are primarily affected by the manner and the rate at which liquid migrates from yarn to yarn. Analysis of the results of ink jet printing of pigment ink on textile fabrics showed that excessive drop spreading and higher line widths were observed where continuous and narrow capillaries prevail on the surface of yarns. Yarn surface characteristics are more important than fabric construction parameters.

This study investigated the transport coefficients involved in mass and heat transfer during the drying of a porous solid partially saturated with multicomponent mixtures.  It included the coefficients governing liquid transport through the solid, the matrix of multicomponent diffusion coefficients in the liquid phase, and the effective thermal conductivity.  As it is not possible to determine these coefficients by theoretical considerations alone and considerable experimental work is required to determine them in a broad range of process conditions, the principle of this study has been the use of mathematical models complemented with some empirical parameters.  These empirical parameters were determined by comparison between measurements in specially designed experiments and the results of mathematical models that describe the process.  In addition, the application of the multicomponent diffusion coefficients is described in two cases where liquid diffusion is important: convective evaporation of a multicomponent stationary liquid film and a falling film. To study liquid transport through the solid, isothermal drying experiments were performed to determine the transient composition profiles and total liquid content of sand samples wetted with ternary liquid mixtures with different initial compositions and temperatures.  A mathematical model including mass transfer by capillary movement of the liquid and interactive diffusion in both the gas and liquid phases was developed.  To simulate the capillary movement of liquid mixtures, parameters experimentally determined for single liquids were weighed according to liquid composition. A fairly good agreement between theoretical and experimental liquid composition profiles was obtained considering that axial dispersion was included in the model. To study the matrix of multicomponent diffusion coefficients in the liquid phase, the redistribution of liquid composition in a partially filled tube exposed to a longitudinal temperature gradient was analysed.  Experimental work was carried out using two main ternary mixtures with different initial compositions and temperature gradients.  Experimental data were compared with the results of a theoretical model that describes the steady-state liquid composition distribution in a partially filled non-isothermal tube to find the empirical exponent that modifies the matrix of thermodynamic factors.  Correlations for the exponents as a function of temperature were determined for each particular multicomponent mixture. The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures was studied by measuring the liquid composition, liquid content and temperature distributions in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side.  Simulations performed at a quasi steady state were compared with experiments to estimate the adjusting geometric parameter of Krischer’s model for effective thermal conductivity, which includes the contribution of the evaporation-diffusion-condensation mechanism. The results revealed that a resistance corresponding to a parallel arrangement between the phases seems to dominate in this case. In the study of the convective drying of a multicomponent stationary liquid film, the equations describing interactive mass transfer were decoupled by a similarity transformation and solved simultaneously with a conduction equation by the method of variable separation.  Variations of physical properties along the process trajectory were taken into account by a stepwise application of the solution in time intervals with averaged coefficients from previous time steps.  Despite simplifications, the analytical solution gives a good insight into the selectivity of the drying process and is computationally fast.  On the other hand, numerical simulations of the convective evaporation of the multicomponent falling liquid film into an inert gas with a co-current flow arrangement of the phases almost always revealed a transition from liquid-phase-controlled conditions to a process in which neither the gas nor the liquid completely controls the evaporation. The results obtained in this work would be useful in implementing models to improve the design, process exploration and optimisation of dryers by incorporating the solid-side effects to describe the drying of liquid mixtures along the whole process. / QC 20110124