Cutting a drop of water pinned by wire loops using a superhydrophobic surface and knife.

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. Distortion of drop shape by the superhydrophobic knife enables a clean break. The driving force for droplet formation arises from the lower surface free energy for two separate drops, and it is modeled as a 2-D system. An estimate of the free energy change serves to guide when droplets will form based on the variation of drop volume, loop spacing and knife depth. Combining the cutting process with an electrofocusing driving force could enable a reproducible biomolecular separation without troubling satellite drop formation

Images (A-D) show a curve drawn using Equation (5) superimposed on top of an example still from a video.

<p>The curves and droplet profiles correspond closely. All scale bars are 1 mm. (<b>A</b>) A droplet with a volume (<i>V_s</i>) of 60 µL and with separation distance (<i>h_o</i>) of 8.5 mm. The normalized time parameter, <i>B,</i> from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045893#pone.0045893.e005" target="_blank">Equation (5</a>) is set to 0.70 for this curve. The parameters <i>A</i> and <i>C</i> from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0045893#pone.0045893.e005" target="_blank">Equation (5</a>) are both set to 1 in all images for simplicity. (<b>B</b>) <i>V_s</i> = 60 µL, <i>h_o</i> = 10.5 mm, and <i>B</i> = 0.75 (<b>C</b>) <i>V_s</i> = 60 µL, <i>h_o</i> = 10.0 mm, and <i>B</i> = 0.30 (<b>D</b>) <i>V_s</i> = 60 µL, <i>h_o</i> = 10.0 mm, and <i>B</i> = 0.40. Images (<b>C</b>) and (<b>D</b>) are of the same droplet.</p

Upper Limit Predictions and Results.

<p>Examples of upper limit predictions as compared to ImageJ measured values and observed drop cutting for a zinc coated superhydrophobic knife and a Teflon superhydrophobic surface (based on the 2-D model). Out of 15 videos capturing drop splitting, there were 74 photos analyzed. In 17 of them (23%), the measured values for h_top exceed the calculated upper limit, however all of those were still within 7% of the calculated upper limit.</p

Schematic contrasting splitting a drop by pulling each end with the method of cutting with a superhydrophobic knife.

<p>Both drops lie on superhydrophobic surfaces. A simple rectangle can estimate the shape and contour of the drop (<i>h_top</i> for the top contour of the stretched drop, and <i>h_o</i> for the bottom contour of the stretched drop). The split drop results in two equally sized spheres, both of radius <i>r_d</i> (equal to the radius of the wire loops used to pin and stretch the original droplet).</p