Change In Apparent Contact Angle Research Articles

Numerical study of droplet evaporation on heated flat and micro-pillared hydrophobic surfaces by using the lattice Boltzmann method

The evaporation processes of sessile droplets on both heated flat and structured hydrophobic surfaces are numerically studied by using a thermal multi-phase lattice Boltzmann model. The contact-line dynamics and heat transfer regarding the evaporation of droplets are simulated and the effects of the heat conductivity, wettability, and roughness of the substrate on the droplet evaporation are investigated. The results indicate that the different evaporation modes of the droplets on the rough surfaces are attributed to the microscopic “stick–slip” motion of the contact line. The constant contact angle (CCA) mode of droplet evaporation on the rough hydrophobic surface can be regarded as a macroscopic average of the microscopic “stick–slip” processes with a short stick period and small change in apparent contact angle. The weaker stick effect of the surface with a higher hydrophobicity and smaller solid fraction makes the droplet evaporation more likely to occur in the CCA manner.

Structural evidence for the timescale separated liquid imbibition phenomenon in porous media

When observing supersource wetting during liquid imbibition into complex fine porous media, it is noted that the experimental imbibition rate at the initial stage after liquid contact is frequently greater than that of the subsequent longer term imbibition. This is in contradiction with classical imbibition theory assuming a single equivalent smooth wall horizontal capillary of known radius and given liquid-solid contact angle (Young-Laplace equilibrium). The essentially two timescale behaviour was highlighted in earlier work by the same authors using a paper coating formulation consisting of ground calcium carbonate pigment particles with increasing amount of styrene acrylic binder. The internal surface structure of pores was assumed to generate a pore wall rugosity impacting, in turn, on the length of the liquid-solid wetting contact line. In this paper, we set out to parameterise the internal structure of the pores by nitrogen sorption, and so provide a direct analytical correlation with the observed imbibition rate change between the short timescale and the longer timescale absorption. The effect of internal roughness can be captured in respect to apparent liquid-solid contact angle. The result for a given internal surface roughness is equivalent between the two approaches, in that the contact line length extension as a function of increased surface structure area, formerly assuming constant contact angle, is simply re-expressed as the change in apparent contact angle due to the surface roughness. The structure equivalence is considered as a series of ultrafine surface pores or pathways on the mesoscale superposed on the larger volume controlling pores, which can also promote capillary condensation as the wetting front follows the vapour phase acting within the internal surface pore structure of the porous medium once imbibition is established.

Thermal and Mechanical Effects in the Spreading of a Liquid Film Due to a Change in the Apparent Finite Contact Angle

A new physical model for the spreading dynamics of fluids with an apparent finite contact angle on solid substrates is presented. The model is based on the premise that both interfacial intermolecular forces and temperature control change-of-phase heat transfer and (therefore) motion in the moving contact line region. Classical change-of-phase kinetics and interfacial concepts like the Kelvin–Clapeyron, Young–Dupre, and augmented Young–Laplace equations are used to compare the effects of stress (change in apparent dynamic contact angle) and temperature (superheat). Explicit equations are obtained for the velocity, heat flux, and superheat in the contact line region as a function of the change in the apparent contact angle. Comparisons with experimental data demonstrate that the resulting interfacial model of evaporation/condensation not only describes the “apparently isothermal” contact line movement in these systems at 20°C but also describes the substrate superheat at the critical heat flux.

Mechanical and thermal effects in the forced spreading of a liquid film with a finite contact angle

Abstract A Kelvin—Clapeyron model for the spreading dynamics of simple fluids with an apparent finite contact angle is presented. The forced change in the apparent contact angle is a measure of the change in the intermolecular stress field which leads to a change in the vapor pressure (which is also a function of the non-uniform temperature field). Combining this with classical interfacial change-of-phase kinetics, equations are developed that combine stress (change in apparent dynamic contact angle), motion and thermal (superheating) effects in the contact line region.