Brownian motion, the random movement of microscopic particles in liquids and gases resulting from their collision with moving atoms or molecules, was first observed in 1827 by the Scottish botanist Robert Brown, using a microscope to look at pollen grains moving randomly in water. In 1905, Albert Einstein showed that the motion observed by Brown is due to the thermal motion of the individual water molecules. This explanation served as convincing evidence that atoms and molecules exist, and was further verified experimentally by Jean Perrin in 1908. Understanding Brownian motion provides insight into the microscopic world, answering questions on how substances interact with each other at different time scales. It has been challenging to study Brownian motion using conventional techniques due to the fast and random movements of the particles at the very short time scale of 10^-10 s (the so-called ballistic regime). Since Brownian motion is extremely sensitive to temperature variation, nanothermometry, which monitors thermal fluctuation at the nanoscale, may be used for studying the Brownian motion of nanoparticles (see Figure).

Now, a team led by Prof. Luís Carlos from University of Aveiro (Physics Department & CICECO), Portugal, and Prof. Xiaogang Liu, from the NUS (Chemistry Department), Singapore, together with researchers from Nanjing Tech University, China, has demonstrated the use of upconversion luminescence nanothermometry for measuring the instantaneous ballistic velocity of nanoparticles with different sizes and shapes. Measuring the instantaneous velocity of these tiny particles, however, has proved to be a daunting task, as concluded by Einstein in a 1907 paper: “due to the very rapid randomization of the motion, instantaneous velocity would be impossible to measure in practice, at least for ultramicroscopic particles”.

Einstein’s conclusion was now challenged by measuring the instantaneous Brownian velocity of colloidal suspensions using upconversion luminescence nanothermometry. The research team is not aware of any other proven solution for measuring the instantaneous velocity of sub-100 nm Brownian particles. Moreover, their work has verified 1907 Einstein’s prediction that the instantaneous Brownian velocity is independent of particle size and shape, under infinite dilution conditions. A better insight into the Brownian motion of suspended particles in nonequilibrium systems has a considerable impact in a wide range of scientific fields. For example, it allows an improved understanding of thermal conductivity, convective heat and mass transfer in various types of nanofluids.

Optical tweezers are often used to trap particles while measuring their three-dimensional displacements with high temporal and spatial resolution. However, for sub-100 nm Brownian particles, the best time-resolution achieved by optical tweezers, 10^-10 s, is insufficient to measure the ballistic Brownian motion. Moreover, unlike this particle trapping approach, upconversion luminescence nanothermometry does not require extremely diluted nanofluid samples for measuring Brownian motion.

Upconversion nanocrystals can be well-dispersed into various solvents, providing a versatile tool for investigating the ballistic Brownian motion in non-equilibrium systems, in general. The technique now disclosed may lead to a deeper understanding of fluid properties. It can also be extended to biological systems helping to understand the transport of molecules and cells.

C.D.S. Brites et al. “Instantaneous ballistic velocity of suspended Brownian nanocrystals measured by upconversion nanothermometry”. Nature Nanotechnology. (2016).
http://dx.doi.org/ 10.1038/nnano.2016.111