This Knife Cuts Water Drops Perfectly In Two

Cutting water. It sounds like the name of a film about a heroic sailor facing death on the high seas, not a procedure science has recently managed to reliably perform on a regular basis. But such is the feat a team of researchers primarily from Arizona State University in the US has achieved. But why the heck would you want to slice-and-dice liquids anyway?

It seems a bit silly at first. If you want to “cut” a non-solid, you could just pour it onto the ground or, if it’s viscous enough to hold a shape, throw it at a wall. But this is very indiscriminate — if the mixture in question is made up of different parts, they’re likely going to remain combined no matter what you do.

Science already has a number of ways of separating liquids — take blood fractionation via a centrifuge. Unfortunately, these processes take a lot of time and centripetal force isn’t always applicable. That’s where hydrophobic surfaces come into play:

A particular focus is using the unique surfaces to enable the manipulation of individual drops so that a complex mixture can be rapidly and inexpensively resolved into individual components. In general, a major challenge in bio-molecular separations is to separate a large number of key proteins from biological fluids for a variety of clinical and biotechnological applications.

Yes, surfaces “afraid” of water, or in this case, extremely afraid. As the original research paper explains, it’s a matter of surrounding the liquid drop in question with water-repelling surfaces and then carefully separating them with a blade that’s also hydrophobic:

A water drop on a superhydrophobic surface that is pinned by wire loops can be reproducibly cut without formation of satellite droplets. Drops placed on low-density polyethylene surfaces and Teflon-coated glass slides were cut with superhydrophobic knives of low-density polyethylene and treated copper or zinc sheets, respectively.

The volumes of water used in the experiments ranged from 15-70 microlitres (0.000001 or a litre) and the knife was dropped at a rate of 0.35mm/sec. For photo buffs, the images above were taken using a AF Micro Nikkor 105 mm 1:2.8D lens attached to a Nikon Digital SLR D5000.

So while we won’t be seeing super-hydrophobic knives being sold at department stores any time soon, they could well become an excellent tool for scientists and pathologists in the future.

[PLOS ONE, via New Scientist]

Images: Yanashima et al

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