| Droplet jetting plays an important role in the printing of biological cells and tissues,the fabrication and packaging of flexible electronic circuits,and the molding of complex microstructured materials.The traditional thermal bubble,piezoelectric,electrostatic,valve-controlled and mechanical droplet jetting technologies all eject droplet from a micropore,which generally have the problems of narrow application range of liquid materials,complex system structure,easy wear and tear of the device structure,easy clogging of nozzles and cleaning difficulties.The emerging surface acoustic wave(SAW)-driven droplet jetting technology does not require nozzle or hole to confine and accelerate the fluid to form a jet,which can directly focus the acoustic energy on the free liquid surface to achieve droplet jetting,even for almost any fluid types.With the advantages of simple structure,multi-functionality and ultra-miniaturization,this technology must be an important research direction in the future.However,this technology currently lacks the design and analysis method of system parameters,holds the negative temperature gradient problem that leads to short working life for rapid split of the device,and is limited to the droplet ejection phenomenon caused by a single droplet,resulting in poor jetting stability and consistency,and low efficiency.Based on the above questio ns,the theoretical and experimental researches on the formation and deposition characteristics of droplet jetting driven by SAW is of great significance for the development of droplet jetting technology and SAW microfluidic technology.In order to provide stable and reliable SAW-driven droplet jetting experimental conditions,the SAW characteristics and the fabrication and packaging methods of the SAW device were studied.Considering the energy conversion efficiency,frequency response characteristics,stability and consistency,the selection of piezoelectric substrate materials and the structural design of the interdigital transducer(IDT)were completed.The piezoelectric coupling mathematical model and the finite element calculation model were established to optimize the parameters design by analyzing the influence of the IDT parameters on the excited SAW.A sandwitch packaging structure was designed to effectively encapsulate the SAW chip,the PCB and the metal heat sink,which makes up for the defects o f the current SAW devices that are prone to rupture and failure due to temperature gradients.The results show that when the device carries a power of up to 10 W,it can continue keeping stable work exceeds one hour.The S-parameters,temperature characteristics,vibration characteristics and acoustic-induced streaming were tested and characterized to verify the correctness and reliability of the theoretical analysis,and to provide a reliable condition for subsequent droplet jetting experiment research.In order to reveal the inner relationship of the SAW-driven droplet jettiing process,the modeling and formation mechanism of the SAW-driven free droplet jetting were studied.Based on the theory of acoustic streaming,the hydrodynamic model of the interaction between the SAW and the liquid was established.A SAW-driven droplet jetting experiment system was built to observe the possible behaviors of free droplets under the action of SAW,and four droplet jetting states,including no droplet,single droplet,satellite droplet and multiple droplets,were obtained.The finite element calculation model of level set liquid two-phase flow was established to simulate and analyze the process of droplet jetting.The calculation model analyzed the dimensionless parameter conditions(5.3<log(We)<5.8),the formation process of single droplet,the velocity and pressure changes,and the energy conversion relationship during the process of single droplet jetting in detail to explain the formation mechanism of droplet jetting d riven by SAW,which provides theoretical support for the subsequent further adjustment and control of droplet jetting parameters.In order to effectively improve the droplet jetting accuracy,the influencing factors and control methods of the SAW-driven droplet jetting parameters were studied.The factors,mainly including acoustic parameters(RF input power,SAW device characteristic response frequency,pulse action time),liquid characteristic parameters(liquid volume,liquid viscosity,liquid surface te nsion)and solid surface energy parameters(hydrophilic versus hydrophobic),that m ay affect the single droplet jetting parameters,were analyzed one by one.The influence law of each factor on droplet jetting parameters,covering the stretching length of the liquid column,the size of the ejected droplet,the position of the droplet separation point,the droplet jetting speed and the droplet separation duration,were quantified.Integrating the experimental data of droplet jetting and fitting multi-parameter curves to obtain the empirical formula for the jetted droplet size,fills the current blank of uncontrollable droplet size and provides a basis for further improving droplet accuracy in the future.Aiming at the problems of huge workload,low droplet jetting efficiency,and poor controllability and repeatability caused by SAW-driven free droplet jetting,the boundary-constrained droplet jetting characteristics were studied.Two boundary confinement structures,the built-in capillary and the external capillary,were designed,and two-dimensional mathematical models were established respectively.The finite element simulation method was used to calculate the sound pressure distribution characteristics of the liquid in the capillary under the action of the SAWs.The droplet jetting phenomenon was observed experimentally to verify the rationality of the simulation analysis and prediction results.The effects of capillary size,liquid level heights and initial liquid state on droplet jetting were studied,and the necessary conditions for stable droplet jetting rwere evealed.Based on the results of simulation and experiments,a continuous liquid supply droplet jetting system was established,and a highly consistent droplet jetting with a coefficient of variation of 1.63% was achieved.For the purpose of effectively improve the printing accuracy and speed,the deposition characteristics and application of the jetted droplets induced by SAW were studied.By analyzing the motion process and energy of the jetted drop let hitting the solid substrate,a multi-physics coupled mathematical oscillation model for jetted droplet deposition was established.Considering the influence of the jetted droplet motion parameters,liquid physical parameters and target base surface characteristics on the droplet deposition process,the adjustment directions of the spreading interval and time feed when the jetted droplet parameters change were given to avoid mutual interference during deposition.A droplet deposition platform was built,and the SAW-driven droplet jetting and deposition system was applied to the hydrogel printing of human umbilical vein endothelial cells,and the cell viability was as high as 97.1%,which lays the foundation for the application of the SAW-driven droplet jetting in bioengineering. |
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