An international group of scientists led by Alexander Zakharov, the IPMash RAS Chief Researcher, discovered that the heating direction of a microchannel with a liquid crystal changes the flow velocity by almost five times. The results of the work are published were Crystals.

Microfluidics is one of the fastest growing fields of modern science. The ability to control the movement of tiny volumes of liquid in channels several micrometers thick opens the way to the creation of «laboratories on a chip», highly sensitive medical test systems and a new generation of optical devices. Until recently, the main tool for controlling a liquid on such a scale was an electric field.

An international team of scientists led by Alexander Zakharov, the Chief Researcher of the IPMash RAS Laboratory of Mathematical Modeling of Wave Processes, proposed and theoretically justified an alternative method — using the temperature difference between the walls of the microchannel. In particular, the technology uses liquid crystals, a unique state of matter that combines the fluidity of a liquid with the ordered structure of a solid. Their elongated molecules can line up in a certain direction like a soldier's formation, but at the same time they can rearrange themselves under external influence. It is this sensitivity that has made liquid crystals the basis of modern displays.

However, research has shown that their potential is much broader. It turned out that in micro-volumes, liquid crystals react extremely not only to electricity, but also to the temperature gradient — the temperature difference between the upper and lower walls of the channel. Moreover, this reaction turns out to be unexpectedly complex and largely depends on the boundary conditions, that is, on exactly how the molecules are «bound» to the bounding surfaces.

«We built a computer model of a compressible liquid crystal enclosed in a flat microchannel and studied how the flow occurs in it when heated from above or below. Calculations show that the direction and strength of the hydrodynamic flow depend strongly on which wall is hot and which is cold. When the sample was heated from above, the flow horizontal velocity reached about 35 micrometers per second. When the heat flow was directed from the bottom up — with the same temperature difference — the speed dropped almost five times, to 6.6 micrometers per second” — said Alexander Zakharov, the Chief Researcher of the IPMash RAS Laboratory of Mathematical Modeling of Wave Processes.

Moreover, in both cases, not only horizontal but also vertical currents formed in the channel, creating a complex three-dimensional pattern of movement. This means that by simply changing the heating direction, one can control not only the velocity but also the flow structure itself.

The development allows us to deal with the same challenges as in microelectronics. Silicon transistors have approached the physical limit of miniaturization, and further progress requires new principles. Similarly, controlling the micro-volumes of liquids using electric fields has its limitations, especially when working with biological samples. Use of a temperature gradient opens the way to simpler, cheaper, and more energy-efficient micro pumps, mixers, and sensors.

Alexander Zakharov emphasized that the work shows not only useful ways to estimate relaxation times, but also allows for analyzing problems of stability, efficiency and lifetime of future devices.