Lattice Boltzmann simulation of droplet dynamics on granular surfaces with variable wettability

Model of drop infiltration into a thin amphiphilic porous medium

HYPOTHESIS

Water drop infiltration into a thin amphiphilic porous medium is influenced by wettability. Due to the reorganization of amphiphilic matter in contact with water, polar interaction changes the wettability in the bulk porous medium and at the liquid/porous substrate interface. To model out of equilibrium water transfer, we propose a thermodynamics approach derived from Onsager's principle.

MODELING

A 2D macroscopic gradient-dynamics model coupling the drop infiltration and the water dynamic into an amphiphilic porous medium is developed and applied to rhizospheric soil in presence of exopolysaccharides (EPS) as an example. The free energy of the entire drop and porous medium system is defined by taking into account the free surface energy of the water and the effective interaction between the porous matrix and the amphiphilic matter.

FINDINGS

The temporal evolution of the 2D drop volume and contact angle are studied during infiltration using the new formulation. Depending on amphiphilic concentrations and initial water saturation, numerical simulation captures similar scenarios to those described in the literature for powder media, as well as a latency phenomenon occurring in dry soil. The latter has been until now poorly modeled.

The Impact of Nanofluids on Droplet/Spray Cooling of a Heated Surface: A Critical Review

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified.

Experimental Investigation of Water Droplet Impact on the Electrospun Superhydrophobic Cylindrical Glass: Contact Time, Maximum Spreading Factor, and Splash Threshold

The impinging of water droplets on superhydrophobic cylindrical glasses has been investigated experimentally by using a high-speed camera. The superhydrophobic cylindrical surfaces were fabricated by electrospinning technique combined with silane treatment. The effects of the diameter ratio of cylindrical glass and Weber number on the postimpact regime, contact time, maximum spreading factor, and splash threshold were investigated in the ranges 3.5-16 and 27-161, respectively. The results were compared with impact droplets on superhydrophobic flat glass and uncovered hydrophilic cylindrical glass. Three types of regimes were observed on hydrophilic and superhydrophobic cylindrical glasses including coating, splash, and splash-rebound. Results showed that contact time on the cylindrical surface is up to 50% less than the flat one. Moreover, the splash regime was started at the critical Weber number = 134 on high-diameter-ratio superhydrophobic cylindrical and flat surfaces while happening earlier when the diameter ratio is below D* < 4.

Capture and re-entrainment of microdroplets on fibers

The capture of liquid microdroplets on fibers, webs, and surfaces is important in a range of natural and industrial processes. One such application is the fibrous filtration of aerosols. Contact angle and wetting dynamics have a significant influence on capture and re-entrainment, yet there is no comprehensive model that accounts for these properties and their influence on capture efficiency. In this study, a series of computational simulations using liquid droplets and air are carried out to investigate the influence of equilibrium and dynamic contact angles on the capture and re-entrainment of mist droplets. A range of operating conditions for droplet-fiber diameter ratios, flow velocities, and contact angles, encapsulating both super-oleophilic and super-oleophobic media, are considered. All simulations are carried out using the volume of fluid (VOF) interface capturing approach in the finite volume solver interFoam within OpenFOAM. The physics of microdroplet impacting on a fiber is discussed and three distinct regimes for the spreading of the droplet around the fiber-inertia, capillary, and stagnation pressure controlled-are identified. It was found that the classification of filtration media for any fluid system, rather broadly as philic or phobic, based on the equilibrium contact angle alone may be insufficient for two reasons: (i) the characteristics of droplet-fiber interaction, including capture or re-entrainment, differs significantly over the range of contact angles for both philic and phobic media; and more importantly (ii) equilibrium contact angle plays little role in the initial stages of the droplet-fiber interaction that predominantly dictates the fate of the droplet. On the contrary, it is the contact angle dynamics that influences the initial stages of droplet impact on fibers, while commercial filters are seldom characterized based on this property. The isolated influence of equilibrium, advancing and receding contact angles on the potential mechanisms that can result in full or partial capture or re-entrainment are highlighted. The influence of equilibrium and advancing and receding hystereses are summarized in the form of a capture-regime map that shows four distinct regimes: (i) likely capture, (ii) likely re-entrainment with minimal or no capture, (iii) receding contact angle assisted partial or full capture, and (iv) advancing contact angle inhibited partial or full re-entrainment.