Droplet Spreading And Permeating On Porous Media

The spreading and permeating of the droplet on porous substrates are widely involved in our daily life activities and industrial applications,such as coating,dyeing and printing.The spreading and permeating usually occur synchronously but play different roles in practical applications.However,most of the previous studies focused on the permeating process without spreading,or the coupled process integrated with spreading and permeating.The competitive mechanisms of the spreading and the permeating process are still unclear.In this work,a new method is proposed to tune the spreading-permeating competition of the droplet on the porous substrates.Both the solid and liquid properties are discussed to examine the feasibility of the new method by lattice Boltzmann simulations.The deliveries of this study are to provide methods to manipulate the spreading and permeating separately,which is of practical interest in many industrial applications.A modified two phase model was used to study the spreading-permeating process,which combed the original pseudo-potential LBM model and the non-ideal equation of state.The modified model can reduce the spurious current of liquid/gas interface and simulate the systems with large liquid/gas ratio.The simulation results show that the model liquid/gas ratio range is determined by ?=b/a(a and b are the parameters of non-ideal equation of state)and can be enlarged when ? increases.The density fluctuates around the equilibrium density in simulations,which resulting the numerical instability.Therefore,the practical model capacity is less than the value of theory.Based on the compromise of spurious current and interface thickness,we provide the available ranges of parameters ? and Tr(reduced temperature).The improved pseudo-potential model is adopted to simulate the two-dimension droplets spreading on heterogeneous surface with two equally opposite wettability sections.The droplet was set on the junction of the two sections.With different wettability of contact surface,the left side and the right side of the droplet exhibit significantly different wetting characteristics.The results show that the spreading process can be divided into three stages.In the first stage,both sides of the droplet move forward until the droplet spreading velocity on the hydrophobic surface reaches zero.The spreading velocity on the hydrophilic surface is always larger than that on the hydrophobic surface.In the second stage,the whole droplet moves towards to the hydrophilic surface,and finally the contact line on the hydrophobic surface crosses the junction of the hydrophilic and hydrophobic surfaces.In the third stage,the whole droplet spreads on the hydrophilic surface.The results also show that three different spreading laws are always observed only if the horizontal distance between the centroid of the initial droplet and the junction of the hydrophilic and hydrophobic surfaces is less than 50 lattices.The droplet can move from hydrophobic surface to hydrophilic surface spontaneously.Finally,we study the effects of substrate properties(surface wettability,inside wettability and pore size)and liquid properties(surface tension and viscosity)to spreading and permeating process.The results show that increasing the surface wettability and the porosity contact angle both inhibit the spreading and the permeating process.The droplets are more easily to permeate into the substrate with the small inside porosity contact angle(hydrophilic),as well as the large pore sizes.Otherwise,the droplets are more easily to spread on the substrate surface with the small surface contact angle(hydrophilic)and the smaller pore sizes.Contrasted to the surface wettability,the spreading-permeating process is more sensitive for the variation of inside pore wettability.The permeating process is enhanced by the increasing of the surface tension,leading to the smaller droplet lifetime.Moreover,the variation of fluid properties has fewer effects on the spreading-permeating process than that of solid properties.