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.
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 [
Droplet manipulation mechanisms that have been investigated cover a broad range of physical principles, including electrowetting on dielectric (EWOD) [
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 space without needing any media. The energy can be delivered to any arbitrary locations in space to trigger an event.
This paper reviews optical manipulation mechanisms that have been utilized for manipulating droplets. A brief comparison of these technologies is summarized in Table
Various optical technologies for droplet manipulation.
Platform | Manipulation principles | Targets | Functions and applications | Light sources | References | |
---|---|---|---|---|---|---|
Pure optics | Optical levitation | Balance of an optical scattering force and droplet gravity | Aerosol droplets (about tens of | Droplet levitation, atmospheric physics, chemistry, and hygroscope | Ar+ laser (488 nm, 1.2 W) | Jordanov and Zellner [ |
Optical tweezers | Optical gradient force | Aerosol droplets ( | Droplet trap, atmospheric chemistry and physics (Brownian dynamics), combustion science, and drug delivery | Nd:YAG laser (1064 nm, 5 mW) | Magome et al. [ | |
Optical vortex traps | Optical cage at the dark core induced by the ring of laser intensity | Aqueous droplets in oil ( | Droplet trap, transport, fusion, dynamic control of concentration | Nd:YAG laser to create Laguerre-Gaussian beam | Gahagan and Swartzlander [ | |
Optothermal | Photothermocapillary in microfluidic channels | Optically induced Marangoni effect | Aqueous droplets in oil (pL ~ | Droplet transport, trapping, high-speed sorting, and protein assay | Holographic laser (532 nm, 4 W) | Cordero et al. [ |
Photothermal cavitations in microfluidic channels | Optical breakdown of liquid molecules | Picoliter aqueous droplets in oil | High-speed droplet generation, and fast droplet merging | Q-switched Nd:YVO4 pulse laser (532 nm, 15 ns width, 100 | Park et al. [ | |
Optoelectronic | Optoelectrowetting (OEW) | Optically induced electrowetting | Aqueous droplets in air or oil (pL ~ | Droplet transport, mixing, splitting, dispensing, and integration with an external reservoir | Laser (532 nm, 636 mW/cm2) | Chiou et al. [ |
OET | Optically induced dielectrophoresis (DEP) | Aqueous droplets in oil (nL ~ | Droplet transport, mixing, integration with microfluidic channel, and microwell structures | LCD | Lee et al. [ | |
Combination of OET and COEW | EWOD for droplet and DEP for microscopic particle manipulation | Nanoliter aqueous droplets in air | Droplet splitting for enhancing particle concentration, and single-cell encapsulation | Projector | Valley et al. [ |
Light-driven droplet manipulation technologies are in general based on three basic concepts: (1) direct optical forces, (2) opto-electrical, and (3) optothermal. Figure
Illustration of optical energy transduction pathways used in optical methods for droplet manipulation.
Optical tweezers is an useful technology in several fields of physics, chemistry, and biology [
Optical levitation is a method using a lightly focused light beam to irradiate an aerosol droplet and create a scattering force to balance the gravitation force of the droplet [
A tightly focused laser beam is used to generate large gradient force to form a stable trap for an aerosol droplet smaller than 10
Figure
(a) Experimental setup of a dual trapping optical tweezers system. Reproduced with permission from [
Magome et al. has demonstrated droplet trapping in air using optical tweezers [
Optical trapping using a strongly focused laser beam with a Gaussian profile allows trapping high refractive index objects
However, optical trapping of aqueous droplets dispersed in immiscible oil, which is a situation commonly used in digital microfluidics, cannot be achieved with this simple Gaussian beam, since the refractive index of water is lower than that of oil
To overcome this issue, Gahagan and Swartzlander have demonstrated three-dimensional trapping of low-index particles in water using a single dark optical vortex laser beam [
(a) Intensity profile of an optical vortex beam with beam waist
Optics has been commonly used as heating sources in many biomedical and manufacturing areas [
The Marangoni effect is a phenomenon of liquid movement induced by surface tension difference. Thermocapillary is the Marangoni effect associated with surface tension difference induced by temperature gradient. This thermocapillary phenomenon was first investigated by Young et al. who observed air bubble motion in silicone oil induced by temperature gradient [
Conventional thermocapillary devices typically use microfabricated electrical resistors to generate heat and temperature gradient for droplet transport, trapping, and sorting on trajectories of prepatterned structures [
(a) Superposition of 100 frames from a video sequence showing the motion of seeding particles near the hot spot. Note that the motion along the interface is directed towards the hot spot. Reproduced with permission from [
Recently, Ohta et al. have also reported droplet manipulation driven by the optothermal capillary effect on a light absorbing a-Si:H coated glass substrate using an optical projector [
Laser-induced cavitation is a phenomenon caused by tightly focusing an intense laser pulse in water to generate a rapidly expanding vapor bubble through nonlinear optical absorption [
Schematic of pulse-laser induced plasma formation and the following shock wave emission and cavitation bubble generation in liquid medium. Reproduced with permission from [
Plasma formation
Shockwave and bubble nucleation
Bubble expansion and collapse
Park et al. have demonstrated a high-speed, on-demand droplet generation device using such ultrafast microfluidic flow triggered by pulse laser induced cavitation bubbles. This mechanism is called pulse laser driven droplet generation (PLDG) [
Schematic of a PLDG device that consists of two microfluidic channels connected by a nozzle-shape opening. A tightly focused laser pulse induces a rapid expanding bubble to push nearby water into the oil channel and form droplet. Reproduced with permission from [
Electrokinetics is one of the most commonly used methods for manipulating small-scale particles and microfluidic flows. Electrowetting on dielectric (EWOD) and dielectrophoresis (DEP) are the most effective electrical based mechanisms for droplet manipulation. DEP manipulates droplets by patterning non-uniform electric fields. Droplets do not necessarily contact the electrodes [
Optical control of electrokinetic phenomena is usually through photoconductive materials. Light illumination on a photoconductive layer can generate virtual electrodes to locally modify electric field distribution, which in turn controls the local DEP or EWOD effects for manipulating droplets.
EWOD is an effective method for manipulating microdroplets. It utilizes electrostatic energy stored in the dielectric layer between a water droplet and an electrode to change local surface tension and droplet contract angle. The difference of contact angle at edges of a droplet induces a net capillary force to actuate the droplet [
Young-Lippmann equation describes that the relationship between the contact angle
Droplet actuation on EWOD devices is typically realized by electrical activation of pixilated physical electrodes. Optoelectrowetting (OEW) uses optical images to trigger local electrowetting effects either on patterned digital electrodes or on a featureless photoconductive thin film. OEW solves the issues of complex wiring and interconnection when a large number of electrodes or droplets need to be controlled in parallel without interference.
The concept of OEW was first reported by Chiou et al. [
(a) Schematic of droplet actuation on an optoelectrowetting (OEW) device. (b) Its equivalent circuit model for one unit cell of OEW. Reproduced with permission from [
Droplet transport
Equivalent circuit
Conventional OEW configuration has two main drawbacks. First, it requires a sandwich structure consisting of two glass substrates. This increases the difficulty of interfacing with other microfluidic components. Second, minimum droplet size that can be actuated is limited by the size of digital electrodes as in regular EWOD devices. To overcome these limitations, several different OEW configurations have been proposed. Chuang et al. reported an open OEW configuration enabling droplet actuation on a single-sided photoconductive surface [
Park et al. demonstrated a single-sided continuous optoelectrowetting mechanism (SCOEW) to enable continuous, light patterned electrowetting on a featureless photoconductive surface [
Schematic of a SCOEW device and its equivalent circuit model. Reproduced with permission from [
Another widely applied principle for droplet manipulation is dielectrophoresis (DEP). It refers to an electrostatic force exerted on a field induced electric dipole on a particle in a nonuniform electric field. This force can be approximated as [
Chiou et al. have demonstrated a mechanism called optoelectronic tweezers (OET) to allow light images to pattern non-uniform electric fields to induce DEP forces for microparticle [
Park et al. have recently demonstrated a mechanism, called floating electrode optoelectronic tweezers (FEOET), specifically aiming for manipulating aqueous droplets suspended in electrically insulating oil and air media (Figure
Schematic of a FEOET device. Reproduced with permission from [
Recently, Valley et al. also reported an integrated platform enabling both COEW and OET manipulation on the same chip [
This paper reviews various optical manipulation methods that have been demonstrated so far for manipulating liquid droplets. These methods can be categorized into three types: (1) direct optical force, (2) optothermal, and (3) optoelectrical. Methods using direct optical forces can provide precise trapping of micrometer scale droplets in three dimensional spaces and are ideal tools for fundamental science studies. However, the high optical power requirement limits its throughput and applications in other areas.
Methods using optothermal effects can provide large forces for high-throughput and high-speed manipulation. Localized optical heating can generate temperature gradient to change surface tension, which is the dominating force in the micrometer scale, for droplet manipulation via the Marangoni effect. Optothermal actuation can also be accomplished by using intense short laser pulses to induce explosive cavitation bubbles which can generate large and transient pressure for high-speed microfluidic actuation. This novel actuation phenomenon has been utilized to demonstrate on-demand droplet injection up to 10,000 droplet/sec and on-demand droplet fusion.
Methods using optoelectrical effects could potentially provide a high throughput platform via massively parallel processing a large number of droplets across a large area. The major difference between optoelectrical from optothermal or direct optical force is that the optical energy is used to switch on electrical driving force to drive droplet motion, while in direct optical forces or in optothermal, the optical energy is directly used to power the droplet movement. This allows opto-electrical methods to actuate droplets with much lower optical power and induce minimum optical heating effect across a large area.
Optical methods for droplet manipulation are an emerging field that has already shown great promises for many applications. It provides several unique advantages that are difficult to achieve with other methods. It provides 3D addressing capability without the need of any physical electrical or mechanical interconnects that can be difficult to fabricate. Second, optical methods can deliver the highest energy density far beyond any other physical mechanisms can possibly achieve. If such energy can be properly converted into actuation forces through clever engineering designs, it can realize many novel high-speed droplet manipulation functions. Third, optical addressing provides a method to realize massively parallel droplet manipulation on a low cost and disposable platform.
Despite many promising potential applications for this technology, there are still many challenges that need to be overcome. First, many of the optical experiments still require bulky optics and lasers. Miniaturization of optical systems is critical to reduce footprint size of the integrated system, which is important for broadening future applications. A portable integrated system would be the goal. Most of the optical methods presented are enabling technologies that have to be integrated with other subsystems for real applications. Solving the interfacing issues will also be important. With the rapid progresses in photonics and optoelectronics, low-cost, compact, and high-power light sources, switches, and other components can be realized. More fundamental studies and research efforts need to be put on finding more novel and useful optical actuation mechanisms that have higher optical energy conversion efficiency to droplet manipulation, lower optical power requirement, more reliable devices and materials, and mechanisms that can be easily interfaced with other subsystems such as microfluidic devices.
This work is gratefully supported by an NSF CAREER Award (ECCS 0747950), NSF DBI-0852701, and NSF ECCS 0901154.