Laser-Induced Mixing in Microfluidic Channels

Abstract

We demonstrate a novel strategy for mixing solutions and initiating chemical reactions in microfluidic systems. This method utilizes highly focused nanosecond laser pulses from a Q-switched Nd:YAG laser at lambda = 532 nm to generate cavitation bubbles within 100- and 200-microm-wide microfluidic channels containing the parallel laminar flow of two fluids. The bubble expansion and subsequent collapse within the channel disrupts the laminar flow of the parallel fluid streams and produces a localized region of mixed fluid. We use time-resolved imaging and fluorescence detection methods to visualize the mixing process and to estimate both the volume of mixed fluid and the time scale for the re-establishment of laminar flow. The results show that mixing is initiated by liquid jets that form upon cavitation bubble collapse and occurs approximately 20 micros following the delivery of the laser pulse. The images also reveal that mixing occurs on the millisecond time scale and that laminar flow is re-established on a 50-ms time scale. This process results in a locally mixed fluid volume in the range of 0.5-1.5 nL that is convected downstream with the main flow in the microchannel. We demonstrate the use of this mixing technique by initiating the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and nonfluorescent N-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) to yield fluorescent resorufin. This approach to generate the mixing of adjacent fluids may prove advantageous in many microfluidic applications as it requires neither tailored channel geometries nor the fabrication of specialized on-chip instrumentation.

Dynamics of laser-induced bubble pairs

“…This feature may be useful for creating a shear flow near bubble 1. The complicated flow generated by the jetting of the antiphase bubble pair may also be used for laser-induced mixing in microfluidic systems (Hellman et al 2007) by extending their single-bubble scheme to bubble pairs.…”

Shearing flow from transient bubble oscillations in narrow gaps

“…These bubbles can be generated with a focused laser pulse [1][2][3], acoustically excited capillary waves [4], or through spark discharges [5]. Applications of these transient pulsating flows span cell stretching [6,7], liquid pumping [8], switching and sorting [9,10], mixing [11], and droplet generation [12].…”

Fluid Flow and Mixing Induced by AC Continuous Electrowetting of Liquid Metal Droplet

“…However, the microfabrication process involved in this approach is sophisticated and considerably challenging. Alternatively, active microfluidic mixers utilize external energy to stir or agitate the fluid flow to promote the mixing performance, such as, acoustic/ultrasonic [17,18], electro-hydrodynamic [19][20][21][22][23], magnetic [24,25], and laser techniques [26][27][28], et.al. Among them, electrokinetic flow has been extensively employed in microfluidic devices to accomplish pumping and mixing, which refers to generate the flow stream instabilities on the surface of the micromachined electrodes activated by electric field [29].…”