Study On The Motion State Of Micro Particles At Gas Liquid Interface Under The Effect Of Surface Tension Gradient

The gas-liquid interface is an important place for the enrichment of micro particles.The manipulation of micro particles at gas-liquid interface has broad application prospects in the fields of micro assembly,micro mixing and micro cleaning.In the micro scale,the adhesion phenomenon of conventional contact control is prominent and easy to cause pollution.By applying the gradient force generated by the non-uniform external field for non-contact control,the adhesion phenomenon can be effectively avoided,but these forces such as light field gradient force,magnetic field gradient force and electric field gradient force are proportional to?L³?L³and L⁰respectively,once the scale is small,the control efficiency is low.The surface tension(?L^-1)plays an important role in the micro particles floating on the gas-liquid interface,which becomes more significant with the decrease of the scale.Therefore,the efficiency of micromanipulation can be significantly improved by surface tension.Therefore,based on the two ways of surface tension gradient generation,surface tension coefficient gradient and interfacial curvature gradient,the surface tension gradient generated by the temperature gradient induced by Gaussian light and the interfacial curvature gradient induced by capillary wave are proposed to drive micro particles.In this paper,experiments and numerical models are used to study that when UV light with Gaussian distribution is incident on Fe₃O₄droplet with photothermal effect,temperature gradient will be formed on the surface of the droplet due to the uneven distribution of light intensity,and Marangoni convection will be generated to drive the particles to form eddy motion inside the droplet,its maxium velocity can reach the order of?10mm/s.The temperature distribution on the surface of the droplet and the flow state inside the droplet are given in detail.The influence of different droplet height and different profile Gaussian light on the driving effect is analyzed by numerical model.The results show that:(1)with the droplet height decreasing,the driving effect is better;(2)under the same light intensity,the Gaussian light with“thin and high”contour energy concentration has better driving effect.Secondly,based on the vortex motion induced by Gaussian light,a non-contact mixing mechanism in micro droplets is proposed.When Gaussian light is incident vertically from the left and right symmetrical positions of droplets alternately,vortices of different sizes will be formed in the droplets alternately to achieve the purpose of mixing enhancement.The influence of droplet height and Gaussian light incident position on the mixing effect is analyzed.Finally,the influence of temperature rise and light transmittance on the mixing mechanism is discussed.Finally,through the study of the driving effect of capillary wave generated by bubble growth and collapse on the micro particles on the interface,it is observed in the experiment that when a capillary wave sweeps the particles,the particles are pushed forward and pulled back in turn,and produce a significant net displacement,and the maximum velocity of the particles can reach the order of?100 mm/s.Based on this experimental phenomenon,a two-dimensional axisymmetric model is established in COMSOL to numerically study the propagation of capillary wave.The results show that the propagation velocity of capillary wave is in the order of?m/s,and the amplitude and wave velocity decrease continuously due to viscous dissipation.Then,a three-dimensional model is established in COMSOL to study the capillary wave driven interface micro particles,the two-phase flow phase field method is used to simulate the phase interface,and the moving grid interface is used to simulate the particle movement.Because the surface tension is added to the N-S equation as a volume force in the phase field method,there will be a pressure jump at the interface in the phase field method,and the pressure jump value decreases with the increase of the interface thickness.Therefore,the driving effect of capillary wave on particles with different interface thickness is studied.The results show that the particles can move forward and backward only when the interface thickness is thin.When the interface thickness is thick,the particles can only be pushed forward by capillary wave but not pull back.Finally,the capillary wave driving research on particles of different size,different wave source distance and different initial amplitude is carried out,the effects of particle size and mass,energy attenuation in capillary wave propagation and the relationship between wavelength and wave velocity on particle driving and are analyzed.