Imploding bubbles mix fluids on a chip

Fluids travelling through micro channels could be mixed together by being whipped into a laser-induced froth, say Dutch and US researchers. Their technique may provide a simple way to control chemical reactions in a lab-on-a-chip, doing away with the need for pumps, valves, or complicated channel patterning.

When fluids are confined to small volumes, they flow slowly and smoothly with little turbulence, so it is hard to mix them together. But groups led by Claus-Dieter Ohl at the University of Twente, the Netherlands, and Vasan Venugopalan at the University of California Irvine, US, have exploited the well-known phenomenon of controlled cavitation, putting to good use the shockwaves created by collapsing bubbles. 

 As Venugopalan explained, focusing a pulsed nanosecond laser into any fluid creates a short-lived plasma bubble which quickly expands and implodes, causing turbulence around it.

When the bubbles are confined to micrometre-scale channels, Ohl has shown, surrounding fluids flow rapidly in complicated eddies, and mix in microseconds. Venugopalan has already used this effect to initiate chemical reactions.

The advantage of this laser-induced mixing to start lab-on-a-chip reactions, notes Venugopalan, is that it doesn’t need special instruments (to generate ultrasound or electromagnetic fields) on the chip to help fluids mix; nor do microfluidic channels have to be carefully patterned or valved to make fluids run into each other.

 Instead, a series of lasers can simply be directed to any spot where mixing or fluid acceleration is needed. The ambition isn’t unrealistic, as lasers are already being used in a more complicated fashion as tweezers to guide pre-formed liquid bubbles around chips. Unlike ultrasound, a laser can be focused onto a very small spot, so that only a localised liquid stream, about a hundred micrometres across, is disrupted. And a laser can be switched on and off to provide mixing at specific times.

Although the pulsed laser is very intense, it doesn’t generate much heat. Venugopalan estimates that if the entire energy of the laser  pulse is focused into one nanolitre of fluid, the resulting temperature rise is no more than five degrees Celsius. Though some fluid is vapourised by the plasma reaction, a thousand times more is usefully mixed, he says.

Cavitation can also create pores in cell membranes, through which large molecules could be delivered. So both Ohl and Venugopalan hope to combine lysing cells with microfluidic mixing to assist the lab-on-a-chip study of biochemical reactions. 

Richard Van Noorden

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