Microscopic particle manipulation for screening operations

Controlling the movement of microscopic particles is tough; controlling the movement of thousands of tiny particles is a challenge indeed. Chemical and biological screening are two applications that would benefit from a technique which involves the manipulation of large numbers of droplets contained in a small quantity of fluid, and researchers in the US and Austria have recently demonstrated that they can steer those tiny droplets within an electrically insulating fluid using light¹.

Optical tweezers trap particles within the focal point of a focused laser beam due to the high electric field intensity there, but this only works well for single particles. Manipulating multiple particles using electric field gradients can be achieved using conventional electronics, however the fabrication of complex electrode patterns makes this technique a costly one. The recent demonstration holds promise for a far cheaper and more versatile method for the controlled conveyance of large numbers of particles; it is called floating electrode optoelectronic tweezers (FEOET).

The sample consisted of de-ionized water droplets immersed in corn oil, which was housed in a transparent container within a 1 cm wide channel, beneath which was a photo-conductive surface fabricated from amorphous silicon, on a glass substrate. Two aluminium electrodes either side of the channel were d.c. biased to produce an electric field across the sample.

A balanced electric field existed around the aqueous droplets, which were much more conductive than the oil in which they were immersed, until the photo-conductive surface was exposed to light. This broke that balance and caused the droplets to move away from the light toward regions where the electric field was more intense. 681 μm sized droplets were shown to be repelled by laser light projected with an average intensity of just 4.1 μW/cm², moving at a maximum speed of 85.1 μm/s. The light therefore behaved like a virtual electrode.

By projecting light patterns onto the device, it would be possible for thousands of droplets to be individually addressed and manipulated, and by altering the light pattern, the virtual electrodes could easily be changed to perform different functions, something which is far harder to do with physical electrodes. For high throughput operations, the droplets could be processed whilst in the continuous flow of oil through a network of micro-channels, making this a versatile and comparatively cheap technique for biological and chemical processing operations.

¹Park, S., Pan, C., Wu, T., Kloss, C., Kalim, S., Callahan, C.E., Teitell, M., Chiou, E.P. (2008). Floating electrode optoelectronic tweezers: Light-driven dielectrophoretic droplet manipulation in electrically insulating oil medium. Applied Physics Letters, 92(15), 151101. DOI: 10.1063/1.2906362