Study On Wetting Behavior And Stability Mechanism For Fine Droplets/Bubbles

In recent years,with the development of emerging bionics,various biological surfaces have been inspiration for improving the efficiency of industrial activities.Superhydrophobic surfaces of liquids based on lotus leaves and spider silks in nature lead to the design of special surfaces with self-cleaning properties and directional transport.The wettability of substrates and the stability mechanism of the micro-and nano-droplets and bubbles depend on the critical roles of size effect:the smaller the scale of the system,the more influencing factors need to be considered,such as anchoring effects,line tension and interface effects.Therefore,this paper conducts theoretical and simulation studies on the wetting behaviors and stability mechanisms for micro-droplets and nano-bubbles,including following aspects:1.The precise determination of the line tension of sessile droplets still represents a major challenge.At present,the estimates of the line tension from contact angle measurements can differ by 4-5 orders of magnitude.Here we show that the pinning effect of the droplet contact line caused by the substrate inhomogeneities influences the apparent contact angle of the droplet,affecting the determination of the line tension via the corrected Young's equation.We introduce the contribution of pinning effects into the Gibbs free energy differential and derive a modified version of the Young's equation.Using classical density functional theory,we isolate the line tension and the pinning force contributions for substrates with different heterogeneity.The pinning effect leads to metastability of wetting states and influences the contact angle,hence introducing errors in the estimation of line tensions using the traditional analysis of contact angles,based on the modified Young's equation.2.For a bubble on a smooth and rigid substrate,its contact angle is always assumed to be the supplement to droplet contact angle under the same wetting conditions.Here we revisit bubble wetting on smooth solid surfaces via both free energetic analysis and molecular dynamics(MD)simulations.Our study shows a fundamental difference between bubble wetting and droplet wetting:the size dependence of isothermal compressibility of gas bubble leads to a size-dependent bubble contact angle.Based on theoretical analysis we develop a new relation between bubble contact angle and droplet contact angle,which is verified with MD simulations for nano-sized bubbles and droplets.In general,our studies show that for bubbles having a size greater than 10 micrometers,the traditional relation of bubble contact angle being the supplement to droplet contact angle holds.But,when bubble size decreases to several micrometers or even to a size of nanoscale,the relation of contact angle match breaks down,and the deviation from contact angle match increases with decreasing bubble size.3.Fundamental understanding of the wettability of curved substrates is crucial for the applications of microdroplets in colloidal science,microfluidics,and heat exchanger technologies.Here we report via lattice Boltzmann simulations and energetic analysis that microdroplets show an ability of transporting selectively to appropriate substrates solely according to substrate shape(curvature),which is called the substrate-curvature-dependent droplet targeting because of its similarity to protein targeting by which proteins are transported to the appropriate destinations in the cell.Two dynamic pathways of droplet targeting are identified:one is the Ostwald ripening-like liquid transport between separated droplets via evaporating droplets on more curved convex(or less curved concave)surfaces and growing droplets on less curved convex(or more curved concave)surfaces,and the other is the directional motion of a droplet through contacting simultaneously substrates of different curvatures.Then we demonstrate analytically that droplet targeting is a thermodynamically driven process.The driving force for directional motion of droplets is the surface-curvature-induced modulation of the work of adhesion,while the Ostwald ripening-like transport is ascribed to the substrate-curvature-induced change of droplet curvature radius.Our findings of droplet targeting are potentially useful for a tremendous range of applications,such as microfluidics,thermal control,and microfabrication.