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Surface tension plays a central role in the mechanical behavior of soft materials such as gels. Elastocapillary deformation of elastic graded substrates is ubiquitous in soft materials. In this work, the effect of a partially wetting sessile liquid droplet on the elastocapillary deformation of a soft elastic graded substrate is studied. The modulus is assumed to have an exponential form along the thickness direction. By applying the Fourier transformation, a mixed boundary-value problem is reduced into a dual integral equation. The numerical results show that the surface displacement is strongly affected by the inhomogeneity of the material. The study of the wetting properties of gel substrates is essential for both understanding the wetting phenomena of gels and developing gels for applications as soft actuators and sensors that can be used in wearable electronics and soft robotics. [ABSTRACT FROM AUTHOR]

We study global minimizers of a functional modeling the free energy of thin liquid layers over a solid substrate under the combined effect of surface, gravitational, and intermolecular potentials. When the latter ones have a mild repulsive singularity at short ranges, global minimizers are compactly supported and display a microscopic contact angle of π / 2 . Depending on the form of the potential, the macroscopic shape can either be droplet-like or pancake-like, with a transition profile between the two at zero spreading coefficient for purely repulsive potentials. These results generalize, complete, and give mathematical rigor to de Gennes’ formal discussion of spreading equilibria. Uniqueness and non-uniqueness phenomena are also discussed. [ABSTRACT FROM AUTHOR]

We carried out a comprehensive molecular simulation to investigate the evolution of the excess isotherms and the isosteric heat with respect to temperature for argon adsorption on strong and weak substrates. The temperature ranges from well below the bulk triple point temperature to above the bulk critical temperature to show the first- (second-) order transitions and the state of the system at the bulk coexistence pressure P0, whether it is non-wetting, partial wetting or complete wetting (preceded by pre-wetting). It is found that the key parameter that affects the dependence of the transitions on temperature and the state of the system at P0 is the relative difference between the isosteric heat and the heat of sublimation (or condensation). For strong substrates, the state of the system changes from partial wetting to complete wetting when the temperature crosses the bulk triple point temperature, and for temperatures well below the bulk triple point the 2D-condensation occurs in the first and second (and possible higher) layers. For weak substrates, the state of the system changes from non-wetting to complete wetting when the temperature crosses the wetting temperature TW, which is specific to the substrate. For temperatures greater than TW, complete wetting in weak substrates only occurs at pressures close to the bulk coexistence pressure P0 via the initial stage of clustering (unfavourable adsorption), followed by a pre-wetting (known as thin-to-thick transition), and this is reflected in the increase of the isosteric heat from a value less than the heat of condensation λ and approaching λ as the pressure tends to P0. [ABSTRACT FROM AUTHOR]

Single-phase as well as two-phase fluid flows inside mini/micro-channels and capillary tubes are of practical importance in many miniaturized engineering systems. While several issues related to single-phase transport are fairly well understood, two-phase systems still pose challenges for engineering design. The presence of gas-liquid interfaces, dominance of surface forces, moving contact lines, wettability, dynamic contact angle hysteresis and flow in confined geometries are some of the unique features of two-phase systems, which manifest into complex transport phenomena. While Taylor plug/bubble flow is a fairly common flow pattern in several micro-fluidic devices operating at low Bond number, the ensuing transport characteristics are complex and still not fully discerned. This review paper aims at highlighting the nuances and features of a unit cell of a Taylor plug flow, especially focusing on partially wetting systems, which are more common in engineering applications. Emphasis is given to a 'unit cell' flow system consisting of an isolated liquid Taylor plug with adjacent gas phase, confined in a capillary tube. Such a seemingly simple flow condition poses considerable challenges for discerning and modelling local thermo-hydrodynamic transport coefficients. Relevant background information and fundamentals are carefully scrutinized while summarizing the state-of-the-art. The role of wettability and dissipation near the contact line is highlighted via available experimental and simulation results. Local momentum and heat transfer exchange processes during the motion of an isolated plug of partially wetting liquid moving inside a capillary tube are delineated. [ABSTRACT FROM AUTHOR]

We report new analytical solutions for the thickness profile of partially wetting two-dimensional droplets. The model includes the effects of capillarity and both short- and long-range molecular forces. We analyze the dependence of the maximum thickness, the contact angle, and the cross-sectional area on the height of the nanometric precursor film that surrounds the droplet. We found asymptotic expressions for the thickness profile and for the contact angles for large and small droplets. The results are compared to those obtained previously for polar liquids. The analytical solutions found here are useful to assess the validity of the hypothesis and the semi-analytical solutions proposed in the literature. In addition, these solutions enable the inference of information about the molecular potential from the measured steady profiles. [ABSTRACT FROM AUTHOR]

Capillary solutions have long existed for the control of liquid inventories in spacecraft fluid systems such as liquid propellants, cryogens and thermal fluids for temperature control. Such large length scale, ‘low-gravity,’ capillary systems exploit container geometry and fluid properties—primarily wetting—to passively locate or transport fluids to desired positions for a variety of purposes. Such methods have only been confidently established if the wetting conditions are known and favorable. In this paper, several of the significant challenges for ‘capillary solutions’ to low-gravity multiphase fluids management aboard spacecraft are briefly reviewed in light of applications common to life support systems that emphasize the impact of the widely varying wetting properties typical of aqueous systems. A restrictive though no less typifying example of passive phase separation in a urine collection system is highlighted that identifies key design considerations potentially met by predominately capillary solutions. Sample results from novel scale model prototype testing aboard a NASA low-g aircraft are presented that support the various design considerations. [ABSTRACT FROM AUTHOR]

An optical video microscopic technique was used to study hetero-aggregation and selective aggregation phenomena among n-hexadecane oil drops (40–110 µim in diameter) and two types of polystyrene latex particles (6.76 and 30.2 µm, in diameter), suspended inside an aqueous medium with pH varying between 1.1 and 12.9. A single drop was produced in situ using a micropipette inside the aqueous phase-filled glass microcapillary (100–160 µm i.d.) containing the particles. Interactions between the drop and the solid particles and among the solid particles was achieved by movement of the aqueous medium in and out of a second micropipette. Drop-particle interactions were distinctly different from particle-particle interactions. It was observed that the latex particles aggregated irreversibly with the oil drop in all cases except two, viz. for 6.76 µm particles at around neutral pH whereas the irreversibility of aggregation in particle-particle interactions was only seen at the ends of the pH spectrum. At around neutral pH, the floes or clusters of small particles were very weak. Visual observations at each pH are explained on the basis of the classical DLVO (Derjaguin-Landau-Verwey-Overbe ek) theory. Partial wetting of particles surfaces by oil appears to be a key factor in the irreversibility of drop-particle hetero-aggregation. Results indicate that the display of reversible, irreversible or weak aggregation depends on the location and depth of the secondary minimum and that the long-range, attractive, London-van der Waals force is responsible for the initial formation of an aggregate. [ABSTRACT FROM AUTHOR]