| Acoustic droplet ejection(ADE)is a novel method for transferring and handling sub-microliter liquids.It has significant applications in life science research such as synthetic biology,drug development,and personalized medicine with its advantages of non-contact liquid transfer,no-nozzle design,and high throughput liquid delivery.In these applications,the ADE system is not only required to accurately eject droplets with a specific detachment velocity and maintain a good droplet flight trajectory,it also needs to ensure the efficient coalescence of the flying droplets on the target substrate.The ejection of droplets at a specific detachment velocity depends on accurate control of the acoustic input power,but the existing power control techniques require to repeat the tedious power scanning operations for each reagent,which is not suitable for highthroughput liquids transfer.Besides,under different experimental conditions,the upward-flying droplets will have different states after colliding with the target substrate and the sessile droplet on the surface of the substrate.The collision kinetics of upwardflying droplets is not clear,which seriously affects the efficiency of the ADE system.According to these two perspectives,a continuous power control method was firstly proposed to predict the input power based on the liquid properties of reagents.And the effects of droplet collision velocity,substrate wettability and physical properties of liquids on the kinetic behavior of single and double-droplets collisions were studied.Specific research contents are as follows.(1)A method for continuous power control during droplet generation was proposed for the first time.The ADE system was constructed,and droplet excitation experiments were carried out on deionized water,five aqueous sodium dodecyl sulfate solutions with different surface tensions and four aqueous glycerol solutions with different viscosities.The relationship among droplet detachment velocity,liquid properties and input power was analyzed,and a method was proposed to achieve continuous power control of the ADE system using the product of surface tension and viscosity as characteristic parameters.The accuracy of the method in predicting the input power required to obtain arbitrary droplet detachment velocities for different reagents was tested.The experimental results showed that the detachment velocities obtained by using the predicted power to excite the droplets were in good agreement with the expected velocities,and the relative errors were less than 8%.(2)The effect of droplet collision velocity and substrate wettability on the collision behavior of upward-flying droplets was studied for the first time using the ADE system.Through the single droplet collision experiments,it was found that within the power range of stable droplet generation,the excited flying droplets adhere to both hydrophilic and hydrophobic substrates,while for the superhydrophobic substrates,the bounce phenomenon occurs when the droplet collision velocity is greater than 0.40 m/s.Analysis of the collision behavior of the double-droplets reveals four different states,which includes coalescence after minor deformation,droplet bounce,coalescence in the detachment phase and direct coalescence.These four states occur successively with the increase of droplet collision velocity.Through the double-droplets collision experiments on the substrates with different wettability,it was found that the Weber number and Reynolds number of droplets rebounding on the hydrophilic substrate have a wider range,and with the decrease of the substrate wettability,the critical Weber number and Reynolds number for the coalescence in the detachment phase and direct coalescence decrease.In addition,based on the law of energy conservation and Hertzian contact theory in the process of double-droplets collision,the prediction models of the maximum spreading diameter and collision time were established by modifying the droplet morphology in the maximum spreading phase of the bouncing state.And the effects of droplet collision velocity and substrate wettability on the collision behavior of upward-flying droplets were analyzed based on the prediction results.It was found that under the same Weber number conditions,droplet collisions on hydrophilic substrate achieve smaller maximum spreading coefficient and greater viscous energy dissipation,leading them to be more prone to droplet bounce.(3)The effect of liquid surface tension and viscosity on the kinetic behavior of acoustic excited droplets colliding with the target substrate and the sessile droplet was systematically investigated for the first time.Through the single droplet collision experiment,it was found that within the power range of stable droplet generation,droplets with different liquid properties will stably adhere after colliding the hydrophilic substrate.By analyzing the kinetic behavior of the double-droplets collisions with different liquid properties,it was found that the Weber number range for droplet bounce increases as the surface tension decreases,and the critical Weber number for producing the subsequent droplet collision states increases as well.Besides,with the increase of viscosity,the critical Weber number for the droplet collision behavior to transform from the droplet bounce to the coalescence in the detachment phase,and then to the droplet coalescence state increases.Finally,through the theoretical analysis of the energy and deformation in the collision process,the prediction model of the maximum spreading diameter in the direct coalescing state was established by considering the viscous energy dissipation in the quasi-steady state.The theoretical calculation results of the maximum spreading diameter were analyzed,and it is found that the greater the surface tension of the liquid,the smaller the maximum spreading coefficient of the droplet collision;the smaller the viscosity of the liquid,the greater the maximum spreading coefficient.Since the smaller the maximum spreading coefficient of the droplet in the direct coalescing state,the more favorable to obtain stable and quick droplets attachment,the liquid properties and input power could be adjusted according to this conclusion to enhance the stability and velocity of direct droplet coalescent on the target substrate. |
