The induction of the optofluidic force allows the characterization of the nanoparticles in real time

Schemes of the optofluidic force induction scheme (OF2i). (a) The particles are immersed in a fluid and are pumped through a microfluidic channel. A weakly focused Laguerre-Gaussian laser beam with an OAM propagates in the same direction as the flow of the particles and exerts optical forces on the nanoparticles. By monitoring the scattered light from the particles through a microscope objective, information on scattering cross sections is obtained and the velocities of individual particles are obtained by tracking the particles. (b) Simulated trajectories for two selected particles. Due to OAM, the particles move along spiral-shaped trajectories, thus suppressing collisions and blocking of particles in the focus region. (c) The optical force F_{opt, z} and the fluidic force F_{fluid, z} acting on a particle it controls the flux in the direction of propagation z, the optical force F_{opt, x} provides a 2D optical capture in the transverse x direction (the capture force along y is not shown). Credit: Applied physical review (2022). DOI: 10.1103 / PhysRevApplied.18.024056

A team of researchers from Brave Analytics GmbH, in collaboration with a colleague from the Gottfried Schatz Research Center and another from the Institute of Physics, all in Austria, developed a device capable of conducting the characterization of nanoparticles in real time. The group has published their work in the magazine Applied physical review.

Over the past few decades, product engineers have been adding more and more nanoparticles to products to give them the desired qualities, such as for thickening or coloring paints. The types of nanoparticles used depend on many factors, such as their composition and shape, which are generally easily determined. The size of the nanoparticles is also important to ensure consistency, but figuring out how big they are has proved more difficult. An approach called dynamic light scattering has been found to work well, but only with tiny nanoparticles. In this new effort, the researchers have created a device that can be used to determine the size of larger nanoparticles.

The new device is based on the induction of optofluidic force (OF2i). It consists of a transparent cylinder and a laser beam. During use, the cylinder is filled with water in which sample nanoparticles have been added, in this case tiny fragments of polystyrene. The laser is fired in a way that allows the light to travel in a spiral through the water, forming a vortex of water.

Laser light is used in two ways: to push nanoparticles through water and to follow their movement. In such a configuration, the amount of acceleration a given nanoparticle undergoes will depend on its size. Researchers suggest it is similar to a sailboat. Two boats of the same size experiencing the same wind force will be propelled at different speeds if they have different sized sails. And because the laser forms a vortex, the nanoparticles travel in a spiral, making collisions less likely.

The scattered light after bouncing off the nanoparticle can then be observed with a time-lapse microscope, which can reveal the paths followed by the individual nanoparticles. The analysis of the shape of these trajectories can be used to determine the speed variations due to the force exerted by the laser and thus reveal the size of the nanoparticles. Tests showed that the device is capable of measuring nanoparticles in the range of 200 to 900 nm.

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