Optically Controlled Digital Microfluidics: Chemistry in Aerosol and Surface-Microdroplet Arrays

Performing a chemical analysis of a small sample volume is challenging. Recently, new approaches for analysing samples have been developed. Known as lab-on-a-chip devices, these approaches are finding application in a wide range of scientific disciplines including the chemical, biochemical, environmental and biomedical sciences. Miniature flow channels are fabricated in an area comparable to the area of a microchip, permitting the mixing of extremely small volumes of liquid reagents and the analysis of small quantities of sample. Thorough mixing can be difficult to achieve in such small volumes and the chip has a fixed design and cannot be altered once fabricated. An alternative approach has been developed to control and mix liquid droplets a few millimetres in diameter using an array of electrodes, allowing easy alteration and control of the device.We propose here to use light to manipulate an array of liquid droplets. Liquid droplets can be trapped in a light beam, known as optical tweezing. In much the same way as regular tweezers can be used to trap and manipulate particles, a tightly focussed laser beam can be used to do much the same with small particles. Each droplet will be only a few microns in diameter, allowing extremely small volumes to be manipulated and mixed. Also, each droplet can be considered to be a miniature reaction vessel without walls. Using a holographic technique, multiple particles can be trapped and controlled simultaneously. This could permit the parallel control over a large number of small chemical reaction 'vessels', allowing precise control when mixing extremely small volumes of reactants. In addition to examining the control of liquid droplets in air, the control of chemistry in liquid droplets on a surface will be studied.