Table 1: Various optical technologies for droplet manipulation.

PlatformManipulation principlesTargetsFunctions and applicationsLight sourcesReferences

Pure opticsOptical levitationBalance of an optical scattering force and droplet gravityAerosol droplets (about tens of μm in diameter)Droplet levitation, atmospheric physics, chemistry, and hygroscopeAr+ laser (488 nm, 1.2 W)
Nd:YVO4 laser (532 nm, 150 mW)
Jordanov and Zellner [37]
Kohira et al. [36]
Optical tweezersOptical gradient forceAerosol droplets ( 1 1 0 μm in diameter)Droplet trap, atmospheric chemistry and physics (Brownian dynamics), combustion science, and drug deliveryNd:YAG laser (1064 nm, 5 mW)
Nd:YAG laser (532 nm, 2 W)
Magome et al. [40], McGloin et al.[21]
Leonardo et al. [43]
Optical vortex trapsOptical cage at the dark core induced by the ring of laser intensityAqueous droplets in oil ( 1 1 0 μm in diameter)Droplet trap, transport, fusion, dynamic control of concentrationNd:YAG laser to create Laguerre-Gaussian beamGahagan and Swartzlander [47], Lorenz et al. [49]

OptothermalPhotothermocapillary in microfluidic channelsOptically induced Marangoni effectAqueous droplets in oil (pL ~ μL)Droplet transport, trapping, high-speed sorting, and protein assayHolographic laser (532 nm, 4 W)
Argon-ion laser (514.5 nm in vacuum, 159 mW) Projector
Cordero et al. [52]
Vincent et al. [53],
Kotz et al. [54]
Hu and Ohta [55]
Photothermal cavitations in microfluidic channelsOptical breakdown of liquid moleculesPicoliter aqueous droplets in oilHigh-speed droplet generation, and fast droplet mergingQ-switched Nd:YVO4 pulse laser (532 nm, 15 ns width, 100 μJ)
Nd:YAG pulse laser (532 nm, 6 ns width, 100 μJ)
Park et al. [56]
Li et al. [57]

OptoelectronicOptoelectrowetting (OEW)
Optically induced electrowettingAqueous droplets in air or oil (pL ~ μL)Droplet transport, mixing, splitting, dispensing, and integration with an external reservoirLaser (532 nm, 636 mW/cm2)
Laser (760 nm, 15 mW/cm2)
Laser (632 nm, 254 W/cm2)
Projector, or LCD
Chiou et al. [58, 59]
Chuang et al. [60]
Chiou et al. [61]
Park et al. [62]
Optically induced dielectrophoresis (DEP)Aqueous droplets in oil (nL ~ μL)Droplet transport, mixing, integration with microfluidic channel, and microwell structuresLCD
Laser (532 nm, 400 μW/cm2) or projector
Lee et al. [63]
Park et al. [23, 64]
Combination of OET and COEWEWOD for droplet and DEP for microscopic particle manipulationNanoliter aqueous droplets in airDroplet splitting for enhancing particle concentration, and single-cell encapsulationProjectorValley et al. [22]