Effect of holed reflector on acoustic radiation force in noncontact ultrasonic dispensing of small droplets

Abstract

We investigated the fundamental aspects of droplet dispensing, which is an important procedure in the noncontact ultrasonic manipulation of droplets in air. A holed reflector was used to dispense a droplet from a 27.4 kHz standing-wave acoustic field to a well. First, the relationship between the hole diameter of the reflector and the acoustic radiation force acting on a levitated droplet was clarified by calculating the acoustic impedance of the point just above the hole. When the hole diameter was half of (or equal to) the acoustic wavelength λ, the acoustic radiation force was ∼80% (or 50%) of that without a hole. The maximal diameters of droplets levitated above the holes through flat and half-cylindrical reflectors were then experimentally investigated. For instance, with the half-cylindrical reflector, the maximal diameter was 5.0 mm for a hole diameter of 6.0 mm, and droplets were levitatable up to a hole diameter of 12 mm (∼λ).

Ejection of small droplet from microplate using focused ultrasound

“…We can utilize the kinetic effect of ultrasonic waves in handling small objects in air. [15][16][17][18][19][20][21][22][23][24] Although the linear transport, [25][26][27] direction switching, 28) mixing, 29) ejection from the trapped state, 30) and dispensing 31,32) have already been demonstrated to some extent, the ejection of a droplet from a well must be studied to accomplish the goal.…”

A low temperature-rise and facile manipulation method for single micro objects at the air-substrate interface

“…The acoustic method uses the acoustic radiation forces to drive and move micro objects [35][36][37][38][39][40]. It has the merits such as being little affected by material properties of the manipulated objects, little thermal damage to the manipulated samples (in some methods), and simple, flexible and compact device structure.…”

“…The acoustic levitation and transport of particles and droplets in air have been reported [41,42], and design methods for devices have been explored for this purpose. Devendran et al proposed an optimized resonator system that could capture as small as 15 µm particles in air, which employed the acoustic radiation force [39]. Prisbrey used the phased arrays of ultrasound transducers to create 3D dynamic patterns of micro particles in air [43].…”

Acoustophoretic printing

“…However, drop generation requires the liquid to have properties akin to those needed for inkjet printing. To create droplets, surface acoustic waves have recently been used in microfluidic devices ( 21 , 22 ). However, their application in air yields limited control over droplet formation, ejection, and deposition when compared to acoustic printing ( 23 , 24 ).…”