Droplet-based (digital) microfluidics has been demonstrated in many lab-on-a-chip applications due to its free cross-contamination and no dispersion nature. Droplet manipulation mechanisms are versatile, and each has unique advantages and limitations. Recently, the idea of manipulating droplets with light beams either through optical forces or light-induced physical mechanisms has attracted some interests, since light can achieve 3D addressing, carry high energy density for high speed actuation, and be patterned and dynamically reconfigured to generate a large number of light beams for massively parallel manipulation. This paper reviews recent developments of various optical technologies for droplet manipulation and their applications in lab-on-a-chip. 1. Introduction Microfluidic devices promise a broad range of biomedical and chemical applications due to their potentials of small volume requirement, short analysis and diagnostic time, high sensitivity, and high throughput analysis [1–3]. Recently, particular attention has been paid to droplet-based (digital) microfluidic devices since droplets isolated in immiscible oil or air can be free from cross-contamination and dispersion [4]. Numerous droplet-based applications including protein crystallization [5, 6], polymerase chain reaction (PCR) [7, 8], enzyme kinetic assays [3, 9], and synthesis of organic molecules or nanoparticles [10, 11] have been demonstrated. Droplet manipulation mechanisms that have been investigated cover a broad range of physical principles, including electrowetting on dielectric (EWOD) [12, 13], dielectrophoresis (DEP) [14, 15], thermocapillary force [16, 17], surface acoustic wave [18], magnetic force [19, 20], and optical forces [21]. In recent years, many light-driven droplet actuation mechanisms have been demonstrated, aiming to provide more functionalities, flexibility, lower cost, and higher throughput droplet manipulation tools or platforms [22, 23]. In general, optical-based droplet manipulation technologies can provide several unique advantages. First, light can be patterned and reconfigured to provide dynamic images, which in turn provides dynamic control of the triggered physical mechanisms without using complex control circuitry on chip. Millions of optical pixels can be readily generated by commercial spatial light modulators such as a LCD or DMD display to provide control of millions of electrodes in parallel on a low-cost and disposable device. Second, some optical methods can provide 3D manipulation of droplets since light can be propagated and focused in free
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