Ejection of small droplet from microplate using focused ultrasound

Abstract

We discussed an ultrasonic system for single-droplet ejection from a microplate, which is one of the basic and important procedures in the noncontact handling of droplets in air. In this system, a 1.5 MHz concave transducer located below the microplate is used for chasing the liquid surface through a pulse echo method, and also for the ejection of a 1 µL single droplet by the burst of focused ultrasound. We investigated the relationship between the droplet ejection characteristics, the distance from the transducer to the surface of liquid, the material property, and the excitation condition of the focused ultrasonic transducer. It was verified that the optimal position of the transducer was off the focal point of sound pressure by ±1 mm, because the sound intensity had to be controlled to eject a single droplet. Subsequently, we confirmed experimentally that the ejected droplet volume linearly depended on the surface tension of the liquid, and that the droplet volume and ejection velocity were determined by the Webber number, Reynolds number, and Ohnesolge number. In addition, by optimizing the duration of the burst ultrasound, the droplet volume and ejection velocity were controlled.

A digital acoustofluidic device for on-demand and oil-free droplet generation

“…The ejected droplet was recorded and measured by dispensing water and ethanol mixtures of different surface tensions. The dependence of droplet volume on surface tension was measured (blue data points) and fitted with a linear relationship (red curve) as shown in figure 3(a), which is consistent with an earlier publication [54]. Moreover, after choosing a sample liquid (e.g.…”

A Comprehensive Review of Surface Acoustic Wave-Enabled Acoustic Droplet Ejection Technology and Its Applications

“…Concave acoustic transducers [ 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 ] can create focused acoustic radiation pressure with their unique concave design. Droplet diameter is proportional to focal spot size and inversely proportional to sound wave frequency.…”

Microfluidic Jetting Deformation and Pinching-off Mechanism in Capillary Tubes by Using Traveling Surface Acoustic Waves

“…In particular, realizing microfluidic drop-on-demand jetting allows for complete discrete controllability of the whole jetting process, which mainly includes controlling the jetting droplet diameter, velocity, frequency and direction. At present, the types of droplet jetting methods according to the driving modes mainly include pneumatic [5], thermal bubble [6], piezoelectric [7], electromagnetic [8], mechanical [9] and ultrasound focusing [10]. For some microscale device applications, there are some drawbacks in these methods, such as the limited driving force and inconvenient control of air pressure supply in the pneumatic jetting device, the necessary condition for special liquids that can be heated and evaporated quickly in the thermal bubble jetting device, nonlinearity, creep, aging and hysteresis problems in piezoelectric ceramics, limitation in conductive metal liquid driven by Lorentz force in a magnetic field, mechanical wear of moving parts in a mechanical cavity and the highly complicated and costly components in the ultrasonic system.…”

“…Specifically, fluids undergoing jetting should have sufficient inertia to overcome the viscous stress and surface tension acting on the interface of the fluid and surrounding media [14]. Compared to other droplet-generation mechanisms in microfluidics such as pneumatic, bubbling, piezoelectric, electromagnetic, mechanical and ultrasonic vibration [5][6][7][8][9][10], the SAW-microfluidic jetting technology has the advantages of great force, high efficiency, flexible design, simple fabrication, cost-effectiveness, lightweight and miniaturization for easy integration, and relatively wide fluid viscosity range. The droplet jetting phenomenon actuated by SAW was first reported in the last decade of the twentieth century [18], in which radio frequency (RF) power for jetting has been concluded as between the effects of more RF power in simply moving the drop and the effects of less RF power in atomizing the drop.…”