The movement of small particles in a fluid as they collide with the fluid’s molecules are known as Brownian Movemnets. Microscopic particles in liquid or gas undergo Brownian motion – jittery, random movements that are the result of countless collisions with neighbouring molecules.

Details of Brownian Motion

In the year 1907 Albert Einstein studied this motion and predicted that a microscopic particle’s kinetic energy and thus the square of its velocity should be proportional to the temperature of its surroundings.If the position of a particle can be measured rapidly enough, it might be possible to measure its velocity before collisions knock it off course.

But Einstein predicted the short time scales between collisions would make these measurements “impossible”.

A team from the University of Texas at Austin has found a way to do it at least in air. The density of air is lower than water, so collisions are less frequent and microscopic particles change direction on longer time scales. To measure the velocity, the team used two laser beams to trap a dust-sized, 3-micrometre-wide glass bead in mid-air. By measuring how much the laser light was deflected by the glass bead as it moved around, the team could make multiple measurements of a particle’s position before collisions caused it to veer off course. These position measurements enabled them to obtain a measure of the velocity every 5 microseconds and directly demonstrate that the Equipartition Theorem holds.

But directly testing this idea, which is called the equipartition theorem, is difficult to do for Brownian particles. That’s because the many collisions experienced by the particle cause it to change speed and direction extremely quickly.

It is certainly an important achievement to be able to directly measure the velocity of the Brownian particle at these short times it is now becoming possible to track individual particles with very high time and spatial resolution, limited in the end only by how many photons per second one can get to interact with the particle.

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