Study Of Droplet Dynamics On Vibrating Surfaces

The interaction of droplets with solid surfaces is common in nature and daily production and life,which is closely associated with many important applications,such as self-cleaning,spray cooling,liquid transportation,and power generation.Although many studies have been carried out by national and international scholars on droplet dynamics on stationary surfaces,vibrating surfaces are often prevalent in practical scenarios,as manifested by airy droplets colliding with the fluttering insect wings and sprayed pesticides depositing on swaying crop leaves.Besides,imposing the specific mode of vibration on solid surfaces can promote the Cassie-Wenzel wetting transition,enhance the heat and mass transfer,and generate droplet atomization/ejection,which has been widely used in material preparation and ultrasonic cleaning.Nevertheless,the mechanism by which vibration affects the droplet behaviors including spreading,retraction,and bouncing is still unclear.To solve the above problems,the study of droplet dynamics on vibrating surfaces was carried out.Based on the application scenarios such as ultrasonic cleaning and spraying,and the test requirements of droplet dynamics,vibration platforms with frequencies up to 10kHz,15kHz,20kHz,and 28kHz were developed.The composite amplitude transformer(CAT)of the vibration platform was connected to the test surface,which excited the test surface to form harmonic vibrations.The theoretical model of the CAT was established,and simulation analysis and optimized design of the CAT were carried out.It was found that the necking transition section on the output rod of the CAT could weaken the effect of transverse inertia,increase the amplification factor and shape factor,and reduce the local stress at the fixed end of the CAT.After theoretical analysis and optimized design,the developed vibration platforms and their CATs could meet the test requirements of droplet dynamics.Using the vibration platforms,highspeed camera,and other pieces of equipment,the dynamics of the single droplet on vibration surfaces,including spreading,retraction,bouncing,and atomization,were further studied.The main work is as follows:(1)Droplet spreading on vibrating surfaces was studied.It was found that vibration could promote the maximum droplet spreading on hydrophilic surfaces and hydrophobic surfaces.In analogy with the Weber number We,a new vibration Weber number We was proposed to characterize the competition between the vibrationinduced dynamic pressure and the surface tension.Based on the Weber number and the vibration Weber number,the maximum spreading factor βm of droplets on both vibrating surfaces and stationary surfaces was unified into one integrated physical model.In this model,the maximum spreading factor of droplets on vibrating surfaces depended on the synergy of θ,We,and We*,and the effect of vibration on promoting droplet spreading could be quantified by the scaling number expressed as We*/[We1/2sin(θ/2)].The scaling number illustrated the relative importance of vibration-induced dynamic pressure compared to inertial force and surface tension of droplets,which revealed the synergistic mechanism of liquid inertia,surface wettability,and vibration characteristics affecting droplet spreading.(2)Droplet retraction on vibrating surfaces was studied.It was found that the maximum retraction velocity of the droplet on hydrophobic surfaces,Urm,was positively correlated with the Weber number We and the vibration Weber number We*.In analogy with the maximum spreading factor βm,the retraction factor βr was proposed to analyze the droplet retraction behaviors.Based on the law of conservation of mass and energy,a simple and effective physical model was proposed to predict the maximum retraction velocity Urm.It was demonstrated that the maximum retraction velocity of the droplet on vibrating surfaces and stationary surfaces obeyed the universal scaling law Urm2～3γ(1-cosθr)(βm2-βr2)/(2ρR0),where γ,θ,ρ,and R0 are surface tension,receding contact angle,liquid density,and initial radius.In addition,droplet jetting during retraction was studied.It was found that vibration could destroy the droplet’s cavity and inhibit the droplet jetting during retraction,and the onset of the ejected micro droplet depended on the jetting Weber number WeT.(3)Droplet bouncing on vibrating surfaces was studied.It was found that vibration could inhibit the droplet bouncing on the superhydrophobic surface with patterned microstructures.On stationary surfaces,the dimensionless contact time was close to a constant independent of the Weber number We.But on vibrating surfaces,the dimensionless contact time increased with the vibration Weber number We*.It was demonstrated that the increase of the dimensionless contact time and the inhibition of the droplet bouncing on vibrating surfaces were caused by the vibration-induced Cassie-Wenzel wetting transition.Due to the vibration-induced dynamic pressure,the wetting state of droplets on superhydrophobic surfaces changed from Cassie state to Wenzel state,which increased the viscous resistance during droplet retraction and inhibited the retraction and bouncing of droplets.In addition,a new vibration-induced bubble capture was found during the wetting transition,which was caused by the expulsion of the air trapped on the superhydrophobic surface.(4)Droplet atomization on vibrating surfaces was studied,and the wetting transition of droplets before atomization was analyzed.It was found that the contact line radius of droplets depended on the vibration Weber number We*,independent of the vibration frequency,which indicated that the vibration-induced dynamic pressure dominated the wetting transition of droplets.The vibration-induced dynamic pressure changed the internal pressure distribution,leading to the deformation of droplets.In addition,It was indicated that the scaling relation between the threshold amplitude of droplet atomization and the vibration frequency could be expressed as Ac∝[γ/(ρf2)]1/3.On both hydrophilic and hydrophobic surfaces,the mean diameter of the secondary droplets increased with increasing the vibration amplitude and decreased with increasing the vibration frequency.In summary,using the developed vibration platforms,the droplet behaviors on vibrating surfaces including spreading,retraction,bouncing,and atomization,were studied,which revealed the mechanism of vibration affecting droplet dynamics.Droplet dynamics on vibrating surfaces is important for industrial and agricultural applications,such as inkjet printing,thermal cooling,anti-icing,anti-fouling,and pesticide spraying,which will help to promote the interdisciplinary integration and development and solve the problems of droplet control and motion in practical engineering applications.