Dancing droplets under acoustothermal heating

As a novel microheating method, acoustothermal heating utilizes acoustic waves to produce heat on the microscale and enables precise control over droplet position in a microfluidic chip.

Article  |  Spring 2018

Droplet microfluidics is an emerging technology that deals with droplets composed of two immiscible liquids on the microscale. Before its advent, researchers had to repeat laborious experimentation of millions of combinations to identify the optimum of reagents or ingredients, especially in the fields of biochemistry, pharmacology, and material synthesis. However, this innovative technology enables millions of experiments to be performed at high throughput and low cost on a coin-sized droplet microfluidic chip, also known as a ‘lab on a chip’.

In a droplet microfluidic chip, droplets on the picoliter to nanoliter scale serve as independent reaction chambers with isolated microenvironments. Tens of thousands droplets can be produced and processed per second in the automated microfluidic chip without the need of bulky equipment or skilled technicians. For droplet microfluidic applications, precise control over droplet position is crucial. A variety of methods have been proposed for droplet position control within a microfluidic chip in the last decade. However, using the previous methods, droplet position could be crudely controlled only in one direction.

At KAIST’s Flow Control Lab (http://flow.kaist.ac.kr), Professor Hyung Jin Sung and his students recently developed a new microheating method that they call ‘acoustothermal heating’. This novel microheating method utilizes acoustic waves to produce heat and is capable of generating free-form, dynamic temperature gradients on the microscale, which is unachievable using previous methods. Professor Sung’s group applied this unprecedented ability to droplet microfluidics in order to precisely control droplet position.

Under a temperature gradient, droplets migrate due to the interfacial tension gradient induced by temperature variance. When a droplet is exposed to a double-peak temperature gradient, it is acted upon by two counter-balancing thermocapillary forces and, thus, migrates to an equilibrium position. In this regard, precise droplet position control within a microfluidic chip is achieved on the microscale using acoustothermal heating. The developed technology can be utilized for various droplet microfluidic applications toward precise droplet position control.

 

 

An article on this research entitled “Acoustothermal tweezer for droplet sorting in a disposable microfluidic chip” was published on February 21st, 2017 in Lab on a Chip (highlighted on front cover), DOI: 10.1039/C6LC01405D.