Laser-controlled microfluidic systems hold promise for biological purposes and more. How exactly the photons will be used, however, remains a question.
The multiplicity of uses for laser light is tremendous. From semiconductor failure analysis to laser ablation for mass spectrometry, laser light is central to innumerable applications.

Now, investigations into how lasers might be used in microfluidic systems are increasing.

Lab-on-a-chip (LOC) and other self-contained devices are on the horizon and will eventually find their way into the mainstream to reduce the use of reagents in biological applications. Efficient fluid handling technologies will have to develop, too, and some researchers are thinking there might be a better way to move fluids about than by using microscopic pumps and actuators.

However, there is the problem of the liquids themselves. Jon Longtin, an associate professor at the State Univ. of New York, Stony Brook, has investigated the effects of pulsed laser radiation on liquids by using a high intensity laser that could exceed the threshold intensities for nonlinear light-matter interaction in liquids. Liquids, he says “are the most difficult of the common phases that we know. It’s the least understood of the three phases and presents challenges for fundamental theories.”

Longtin’s work notes that at high light intensities, actual liquid behavior can deviate significantly from classical model predictions. In particular, a property called saturable absorption changes drastically at high laser intensities. Absorbed thermal energy, he discovered, can be up to two orders of magnitude less than predicted by classical models.

“Macroscopically, you can measure thermal properties, but from a microscopic point of view, liquids are surprisingly not understood,” Longtin says.

Could this be a hindrance to the successful development of microfluidic systems? Possibly. But researchers are counting on the ways lasers can be used to maximize the efficiency of these systems. According to Longtin, the basic infrastructure of LOCs could be altered by substituting laser control for etching or syringe pumps.

Using newly developed instruments that can capture rapid, microscopic changes in liquid flow using high intensity light, researchers are closing in on some answers.

Spiralling into control
A liquid jet only a few micrometers in width and several hundred micrometers long was caused by radiation pressure from photons generated by a laser beam. Image: Univ. of Bordeaux I.
The quickest way to get from point A to point B is a vortex. At least, that’s the way it appears to researchers who have found a way to use photonic energy to accelerate fluids in low-profile microchannels.

A team of scientists in Europe recently collaborated to demonstrate a way by which cavitation can be instigated with pulsed, high-intensity light. The team includes Ed Zwaan and Claus-Dieter Ohl at the Univ. of Twente, The Netherlands, Severine La Gac of the MESA+ Institute for Nanotechnology, also at the Univ. of Twente, and Kinko Tsuji of Shimadzu Europa, Duisburg, Germany.

The experiment used the 532 nm green light energy of a Solo PIV, from New Wave Research, Fremont, Calif. The Solo PIV is a double-pulsed Nd:YAG laser designed for particle image velocimetry (PIV) to produce bubbles in various types of microchannels. Typically used for flow-based experimentation, the Solo-series laser generates pulses in the 5 to 50 µJ range at a duration of 6 nsec.

The channels used were so shallow, just 20-µm high and 1-mm wide, that they reduced the cavitation problem to a 2-D dynamic. Though highly repeatable, the experiment, lasting just 20 µsec from creation to final decay, required sophisticated observation devices. Among these was a Shimadzu HPV-1 high-speed camera, from Shimadzu Scientific Instruments, Columbia, Md., that has the ability to capture 100 sequential images at one million frames per second.

Tsuji was using the camera to capture cavitation behavior but did not have the PIV capability she needed until she joined other scientists involved in microfluidics research.

“I mentioned I’d like to see the moment when the bubble disappears but didn’t have the equipment to do so. I teamed up with the others and I discovered it’s very difficult to capture micro-bubbles made with lasers,” Tsuji says. To do so, a dye which absorbs laser energy is added to the liquid.

The heat energy of the laser pulse was able to form pancake-shaped bubbles that showed radial properties of expansion and collapse.

“The bubble must be generated much faster than it decays,” says Tsuji. “It depends on laser intensity. Usually, at 1 µsec time duration, a bubble is formed,” which means that despite the camera’s high frame rate, it is too slow to observe the forming process. An ILP-1 arc lamp from Olympus America, Center Valley, Pa., was used along with an image intensifier and variable delay generator to capture the bubble expansion. When used with an optical CF 40 microscope from Carl Zeiss, Thornwood, N.Y., the HPV-1 camera can easily observe the decay, which takes 20 µsec to complete and, in the process, is able to take a sequence of images which can be combined to show a video.

Inertia was a major player in this experiment. A collapsing bubble at a given stand-off distance from an infinite boundary is understood to develop a focused flow. This focused jet is directed toward the boundary, and the impact of flow on the boundary creates a long-lived vortex.In a free environment, the bubbles expand then collapse, emitting a pulse wave that degraded with time. However, when placed in a microchannel chamber, the bubbles show a liquid property known as axisymmetry.

During expansion, the bubbles respond to thermal energy by expanding away from the confinement boundaries, forming geometrically-aligned bulges. As the bubbles reach their growth limit, the bulges flip inward, splitting each bubble into two or more smaller bubbles that exert pressure on each boundary. The result is a jet flow.

In the case of a square-shaped microchannel, two bubbles and two jets of faster-flowing liquid appear and form two counter-rotating vortices, which stir the liquid at high velocities. The bubbles shrink within these two vortices, which last for a considerable duration in comparison to the life of the bubble. Flow force converges in between the vortices and is directed to a much smaller feeding channel.

Other vortex forms are possible. A triangle with feeding channels on each corner will produce three vortices, while a box with two feeding channels at the corner will generate four.The bubbles conform to a 2-D Rayleigh model. The planar flow field during the bubble collapse is measurable, although the bubble does shrink to less than the height of the microchannel and induces a 3-D flow.

According to the research, the observed jetting and creation of vorticity has important practical implications. Microfluidics pumping using laser energy could create vigorous mixing on short time scales.

The preservation of high flow numbers in microfluidics is difficult to do, and lasers provide a way to keep these benefits. Plus, laser light is a noninvasive generation mechanism that circumvents the need for connecting wires or actuators. In addition, the position of the laser spot can be adjusted with mirrors.

The Shimadzu HPV-1, or HyperVision, camera has the ability to capture the rapid sequence of changes during the micro-cavitation process investigated at the Univ. of Twente, Netherlands. Sequences of images can be grouped together to form video. Image: Shimadzu
“We still have a lot of data we haven’t analyzed yet. We did a lot of experimentation, and the next to come will be biological applications. There’s still a lot of work to do,” says Tsuji. Other members of the team are exploring possibilities for medical or biological applications for the technique. The team plans to take the laser research a step further and apply the experiment to the poration of cells in LOC system. According to team member Ohl, the method they are investigating will be similar to electroporation but done with bubbles and pumping fluids by exploiting the jetting mechanism.

This experiment was the first time Tsuji had used the HPV-1 for particle image velocimetry. It’s important to characterize fluidics system on the basis of time, she says, and the success of LOC research depends on the ability to provide tools able to profile these activities.

The power of the photon
Researchers are often forced to abandon more conventional fluids and construct specialized varieties for laboratory work. Nanoparticles and delicate suspensions are crucial to these studies. Water arguably has the most implications for microfluidics systems in biological applications, but its high surface tension and phase change properties make it difficult to control with laser light. Specialized liquids are needed to observe light-liquid interactions in a laboratory, and in using one of these concoctions, a team of scientists at the Univ. of Chicago, Ill., and the Univ. of Bordeaux I, France, made a surprising discovery.

Their work unexpectedly showed that a liquid could be incited to move using radiation pressure alone. Relying on theoretical calculations from Wendy Zhang and Robert Schroll at the Univ. of Chicago, Ill., the French scientists focused an argon laser into a specially designed liquid, resulting in a jet that extended a few hundred micrometers before breaking up into droplets.As Schroll and Zhang suggested, the flow appeared to have been caused by radiation pressure.The laser light exerted a force on the liquid without losing energy to the creation of heat.

“The experiment was originally conceived to demonstrate the effects of radiation pressure on an interface between two fluids. The critical fluid was chosen because the very low surface tension between the two phases means reasonably-sized deformations could be produced at moderate laser powers,” says Schroll.

The fluid in question is a quarternary mixture of which the majority is an oil, toluene. The remainder is water and a surfactant used to synthesize a suspension called a microemulsion. The resultant mixture is made of surfactant-coated nanodroplets of water, called micelles, suspended in oil. The liquid was specifically engineered to provide conditions in which the laser could generate movement.

A fluid like this has a liquid-liquid critical point at about 35°C. Above this temperature, the mixture separates into two phases with different concentrations of micelles. The team wanted to observe light-induced flows from interface deformation, and the interfacial tension vanishes close to this critical point, which is easily achieved in laboratory conditions.

According to Schroll, photons scatter from the fluid with the same energy they had when they entered it. The scattering effect intensifies as the critical temperature is reached, highlighting several other effects at play.

At this critical point the fluid has large fluctuations in its index of refraction. The fluctuations scatter the photons, transferring some of the forward momentum to the fluid and creating a flow.

“The key to this process is the fluctuations in the fluid, not any particular quality of light,” says Schroll. “This research shows that light scattering can produce a bulk liquid flow. Previously, light scattering has been used to move single objects, but we believe that this is the first example of a bulk flow.”

—Paul Livingstone