Employees of the Lebedev Physical Institute conducted an interesting experiment with liquid drops and a special surface with microstructures. It resembles the famous scene from the movie “Terminator 2”. This is reported by “Hitech” with reference to the press service of the university.

One of the most famous scenes of Terminator 2 is the one where a metal drop, moving along the asphalt as if alive, flows to the legs of the T-1000 killer robot and merges with it. Scientists from the Lebedev Physical Institute (FIAN) saw a similar picture in their laboratory and recreated the abilities of the robot from Terminator 2.

In their experiment, liquid droplets spontaneously flowed from place to place on a surface with microstructures “carved” into them with a laser. Such surfaces can be used in microfluidic biochips and medical rapid tests that fit easily in a pocket. An article about the results of the experiment was published in the journal Applied Surface Science.

“Usually, a drop that falls on a flat surface stays in place. We made it move – due to the gradient of surface tension forces. With the help of a laser, we created microstructures on the surface with an increase in its hydrophilicity (wettability), and the drops move along them in the direction where the hydrophilicity is maximum. Such a “horizontal pump”, for example, will allow separating liquids with different surface tension coefficients, simplifying biochips and microfluidic devices,” explains Sergey Kudryashov, co-author of the study, lead researcher and head of the Laboratory of Laser Nanophysics and Biomedicine at the Lebedev Physical Institute.

The technology of pumping water using surface tension energy has long been invented in wildlife. The Texas horned lizard (Phrynosoma cornutum), living in the deserts of North America, has learned to collect and move water that condenses on its body at night. A network of open capillary channels formed by scales causes water to flow directly to her mouth, an effect described by German and Austrian scientists.

To reproduce it in the laboratory, Kudryashov and his colleagues tried to create a surface energy (tension) gradient on the surface – that is, to make the degree of hydrophobicity gradually decrease along the surface from point to point in a given direction. Unfortunately, this cannot be done simply by reducing the thickness of the hydrophobic coating layer on the hydrophilic one. The surface tension force is very short-range, in order to “turn off” the hydrophilicity of the metal, it is enough to apply a layer of plastic one or two molecules thick on it

“You can try to do it chemically, that is, by creating areas with chemically different coatings with different hydrophobicity, but this surface will be very capricious, because any dust, any organic pollution immediately changes the hydrophobicity index, and it is difficult to wash such a surface in order to restore its desired level,” explains Kudryashov.

Therefore, the FIAN scientists decided to take advantage of the fact that a liquid drop has a rather large area and it “averages” the hydrophobicity index in areas with hydrophobic plastic and with hydrophilic metal, where the plastic is removed by laser. In other words, a drop will not be able to distinguish a surface with one hydrophobicity index at each point from a “chessboard” of the same area with different indices in each cell if the average value is the same.

For the experiment, the scientists coated 5cm x 5cm steel plates with a millimeter-sized hydrophobic siloxane-based polymer coating. Then, using a laser with nanosecond pulses, they cut through the coating layer to the metal, creating rows of grooves 5 mm long and about 100 microns wide.

Then, by re-treatment with a laser, the scientists modified them, expanding them to varying degrees. So four sections appeared on the steel plate with different indicators of hydrophobicity – the wetting contact angle, that is, the angle between the surface and the water drop on it conditionally tangent to the surface. On a hydrophobic surface, a drop of water spreads less, so the contact angle will be larger. On the hydrophilic, on the contrary, the angle will be smaller, since the drop spreads more. The contact angle in four areas varied from 46 to 13 degrees.

Then the scientists dripped water on different areas and watched its movement. A drop of water with a volume of five microliters in the experiment spontaneously moved from hydrophobic to hydrophilic areas. The fastest drop moved between the first and second sections – in this place its speed reached 92 mm per second.

Scientists note that such microstructured surfaces can be widely used in the development of microfluidic devices, a rapidly developing field that has already produced dozens of compact devices for studying the chemical composition of air and water, as well as diagnostic medical tests.