Laser microfluidics: fluid actuation by light

Abstract

The development of microfluidic devices is still hindered by the lack of robust fundamental building blocks that constitute any fluidic system. An attractive approach is optical actuation because light field interaction is contactless and dynamically reconfigurable, and solutions have been anticipated through the use of optical forces to manipulate microparticles in flows. Following the concept of "optical chip" advanced from the optical actuation of suspensions, we propose in this survey new routes to extend this concept to microfluidic two-phase flows. First, we investigate the destabilization of fluid interfaces by the optical radiation pressure and the formation of liquid jets. We analyze the droplet shedding from the jet tip and the continuous transport in laser-sustained liquid channels. In a second part, we investigate a dissipative light-flow interaction mechanism consisting in heating locally two immiscible fluids to produce thermocapillary stresses along their interface. This opto-capillary coupling is implemented in adequate microchannel geometries to manipulate two-phase flows and propose a contactless optical toolbox including valves, droplet sorters and switches, droplet dividers or droplet mergers. Finally, we discuss radiation pressure and opto-capillary effects in the context of the "optical-chip" where flows, channels and operating functions would all be performed optically on the same device.

Dynamic interfacial tension effects in the rupture of liquid necks

“…This same system was also used in a microfluidic channel to study the breakup of drops of water in oil (Delville et al 2009). Here the breakup is driven by a co-flow in a cross-shaped junction as shown in figure 1(c).…”

“…A dye (commercial E122 food dye) was added to the water to favour light absorption and locally heat up the water jet. This produces thermocapillary stresses and forces pinching when the beam power exceeds a threshold (Delville et al 2009). By studying the rupture of the jet, after the laser has been turned off, we obtain its thinning dynamics under such conditions.…”

Comparative analysis of nonlinear optofluidic processes in microdroplets

“…The mechanical interplay between light and liquid, however, has not been thoroughly investigated, even though the first demonstration of liquid motion actuated by a focused laser beam was reported as early as 1973 [7]. Since the classic work by Ashkin and Dziedzic, the distortion of a liquid system induced by a high intensity laser beam has been studied in [4,[8][9][10][11][12][13][14][15][16], where both flat and spherical fluid interfaces were considered.…”

“…Our prior work only considered nonlinear effects due to radiation pressure. Yet besides radiation pressure, optical fields can also actuate fluid motions through scattering force [13] and thermocapillary force [19]. In this paper, we compare the magnitude of nonlinear effects induced by the scattering force and the thermocapillary force, as well as radiation pressure induced nonlinearity and other nonlinear effects.…”

Microfluidics

“…-again reducing the cost and time of the complete analytical process and reducing the risk of procedural error. Besides channeling of fluids in capillaries, droplets can be actuated and manipulated using intense light fields [3,4]. These advantages have proven to be very attractive, first spurning a very large scientific activity in the field and increasingly also giving rise to commercial products.…”