Research On Droplet Propulsion By Thermal Power Conversion And Control Of Droplet Dynamics

Microfluidic technology has broad applications in biochemical analysis,synthesis and screening of substances.Droplet manipulation is a key technology in the field of microfluidics.Leidenfrost effect is used to drive droplet and achieve thermal power conversion.However,this method is based on high temperature environment,the working medium is consumed faster,and the conversion efficiency is low.In this paper,a novel droplet driving method is proposed,and the partial boiling mode is used to drive the droplet to oscillate on the superhydrophobic channel,which has the advantages of slow consumption of working medium and high efficiency of thermal power conversion.Inspired by the light-diverging effect induced by convex lens,a new idea of droplet dynamic control was proposed.The one-dimensional linear oscillation motion of droplet is extended to two-dimensional planar oscillation by using convex surface,which makes droplet have potential application value as turbulence element to enhance mixing.The core components of the experimental platform are horizontal superhydrophobic channel and vertical rough side surfaces.When the droplet hits the rough superheated surface on both sides,the explosive boiling generates enough driving force to drive the droplet.The heat transfer mode and thermal power conversion efficiency during droplet collision with side surfaces at different surface temperatures are studied and the dynamics of oscillatory motion of droplet in a linear channel is analyzed.The linear superhydrophobic channel is replaced by a rectangular superhydrophobic channel,and the left and right surfaces are replaced by convex surfaces.The influence of convex surfaces on the droplet dynamic parameters,motion mode,incidence angles and reflection angles during the impact process was studied.With the increase of the temperature of the side surfaces,the impact progresses present three different modes:contact boiling,partial boiling and Leidenfrost.In the partial boiling mode,the explosive boiling generates enough thrust force to drive the droplet,forming oscillation motion and the conversion of thermal energy to kinetic energy was realized.The thermal power conversion efficiency is one order of magnitude higher than that of the droplet drive based on Leidenfrost mode.On the linear channel,the longitudinal oscillation motion of the droplet in the direction perpendicular to the channel axis is observed even the initial velocity of the droplet is along the axis,and the peak value of the longitudinal offset increases with the decrease of the droplet diameter.The longitudinal oscillation motion of the droplet is promoted by the tangential component of the driving force and suppressed under the action of gravity.The relative magnitude of the two forces determine the longitudinal oscillation dynamics of the droplet.The convex side surfaces enhances the longitudinal oscillation motion of the droplet in the rectangular superhydrophobic channel.The longitudinal impact frequency of the droplet is three times that with flat side surfaces,and the longitudinal motion velocity of the droplet is twice that with flat side surfaces.The convex surface greatly enhances the droplet’s longitudinal motion without weakening the droplet’s transverse motion velocity and impact frequency.When the side surfaces are convex,the distribution ranges of incident angle and reflection angle increase during droplet impact progresses,which changes the droplet motion mode and it’s distribution,and strengthens the droplet longitudinal motion.When the droplet hits a convex surface,the tangential component of the contact pressure and the friction force increase the angle of reflection,which is the control mechanism of convex surface on droplet dynamics.The new droplet drive and dynamic control method proposed in this paper provides a new idea for droplet control and mixing strengthening in the field of microfluidics.