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RESEARCH PROFILES Microfabrication with instant gratification down to 200 nm,” says Hunt. Shine a laser on glass, and voilà, a Water “Perhaps we can go smaller, device is born. It may sound too but it starts to get harder as it simple, but as described in the AuBubble gets smaller.” gust 15 issue of Analytical ChemHasselbrink explains another istry (pp 5083–5088), that is just Debris drawback. “This is a serial methwhat Alan Hunt, Kevin Ke, and od where you are writing one E. F. “Charlie” Hasselbrink at the Glass feature at a time. It’s not like University of Michigan, Ann Arbor, lithography where you can have done. By focusing a femtosecDirection of High numerical do an entire wafer at once. ond pulsed laser on a glass surface glass movement aperture objective That’s probably the biggest submerged underwater, the re = 527 nm negative.” However, he adds, searchers created submicrometer Femtosecond “The positive [aspect] is we can fluidic channels under the glass surlaser do almost any arbitrary 3-D face. Additional processing steps were not necessary. A schematic of a new fabrication technique for creating channels shape that you want under the The investigators demonstrated in microfluidic devices. A femtosecond pulsed laser breaks down surface with submicron resolution. As far as we know, nothat their method could produce glass that is submerged underwater. The microbubbles extrude body could do that before.” more than just straight channels. debris, and this results in clean, submicrometer channels. Hasselbrink further points They fabricated a channel in the out that, unlike in lithography, in which that collapse suddenly and violently. A form of a spiral, and they created only one depth per etch is possible, a substantial amount of energy, which usua simple 3-D mixer. ally damages any surface that happens to channel created by their technique needChannels produced by the optical be close by, is released when they collapse. n’t be confined to a single plane. The inbreakdown of glass with a laser have As Hasselbrink explains, microbubbles vestigators have fabricated channels withbeen previously demonstrated (Opt. Lett. in glass that have a gradual incline. are low Reynolds number bubbles. “We 2001, 26, 1912–1914; Appl. Phys. A: The walls of the channels have a can measure the size of the bubbles and Mater. Sci. Process. 2004, 79, 605–612). timescale in which they collapse and infer roughness of ~20 nm. Hunt says that, as However, the laser pulses were high-enthat these are not high Reynolds number expected, the roughness “seems to be ergy and not tightly focused, producing somewhat dependent on [the] machinbubbles,” he says. “They collapse very 10–100-µm-diam channels with rough ing protocol. . . . If the [laser] pulses are slowly compared to the high Reynolds walls and a lot of debris. separated by a fair distance, you’re going Hunt and colleagues used a tightly fo- numbers bubbles that cause damage.” to get a rougher surface.” Although the Hunt elaborates further, “By getting cused, femtosecond pulsed laser, with enroughness is acceptable for the time being, into the low Reynolds number regime, ergy on the nano-Joule scale. The laser the investigators say the smoothness of the you get [the] paradoxical result that was focused on a glass substrate that was walls can be improved in various ways. much lower energy causes you to have submerged underwater. As the glass Hunt and colleagues are now using long-lived bubbles.” That may seem broke down below the surface, “microtheir fabrication method to build miniabackward, he says. “But it makes sense bubbles” were simultaneously produced. ture instruments. Although stumbling that under a low Reynolds number reThe microbubbles gently expanded out gime, inertia of the fluid doesn’t matter. blocks, such as channels getting clogged of the channels and toward the surface. You can neglect momentum, so the bub- by contaminants in samples, have come As they pushed their way out to the surup, the investigators hope to eventually bles don’t overexpand. Because they face, they took the debris from the glass create fully integrated analytical devices. don’t overexpand, they don’t have a with them. The result was essentially Finally, Hunt points out that the fab‘negative’ internal pressure, so they clean submicrometer channels within the rication method is not just limited to don’t collapse violently.” Because the glass substrate. glass and water. The technique works in The investigators realized that the mi- microbubbles don’t cause damage durother liquids, too, he says. “Although ing the machining process, nanoscale crobubbles produced in their method this paper focuses on glass because it’s were different from the bubbles generated resolution is preserved, he adds. the most useful [material], the machining The technique has some limitations. in other micromachining techniques. Bubworks in probably any transparent dielecFor one, the investigators aren’t sure bles produced in micromachining techtric material.” a how narrow they can make their subsurniques, called cavitation bubbles, are —Rajendrani Mukhopadhyay typically high Reynolds number bubbles face channels. “We know we’ve gone S E P T E M B E R 1 , 2 0 0 5 / A N A LY T I C A L C H E M I S T R Y
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