Bending Light Splits Nanoparticles

Scattered Light Separates Nano Particles
Twisted Light Separates Nanoparticles - Twisted light: Self-torque can be used to manipulate molecules. (Permission: iStock/VikaSuh)

Austrian researchers have developed a method to characterize nanoparticles in suspended mixtures. The new method, created by Marko Simi and his colleagues at the University of Graz, pushes nanoparticles into spiral orbits with size-dependent velocities, allowing nanoparticles of various sizes to be studied independently. The result of this new strategy could lead to the processing of nanoparticles.

Twisted nanoparticles article
Acting with the flow: diagram showing the optofluid force induction (OF2i) diagram. The liquid containing nanoparticles flows from left to right in a microfluidic channel, which is also the propagation direction of the bent laser light. (Permission: Marko Šimić et al./Physical Review Applied)

Cosmetics, paper, paints, pharmaceuticals, and a wide variety of industrial processes all benefit from nanoparticles. Controlling the size of nanoparticles is crucial to ensure the highest performance of these products because many of these applications require the nanoparticles to be suspended in a liquid or gel.

Dynamic light scattering, a method based on the random Brownian motion of nanoparticles in a liquid, can be used to achieve this. Brownian motion occurs when nearby molecules shake a nanoparticle, so it is more pronounced for smaller particles. Monitoring changes in light scattered by mixtures of nanoparticles reveals Brownian motion.

Smaller nanoparticles respond relatively well to this method, while larger nanoparticles are less subject to Brownian motion, making their size much more difficult to measure. The approach also lacks the ability to define size in real time, a crucial requirement for contemporary manufacturing processes.

The innovative strategy used by Simi's team is known as optofluidic force induction (OF2i). To do this, a mixture of nanoparticles is pumped through a microfluidic channel in the same direction as a weakly focused optical vortex. The second is a laser beam containing orbital angular momentum and having a corkscrew wavefront around the propagation direction.

The laser beam accelerates particles of various sizes to various velocities, providing a method for assessing particle size in the sample. Particle collisions are common because particles of different sizes move at varying speeds, reducing the velocity separation.

Simi's team used bent laser light to find a solution to this problem. As a result, the nanoparticles gain angular momentum and begin to move in spiral patterns. Collisions are avoided because particles of various masses follow different trajectories.

Using a microscope placed at the bottom of the channel, Imi and his associates tracked the movements of individual particles by detecting the light scattered by the spiraling nanoparticles. They were then able to calculate the velocities of the associated nanoparticles from the shapes of these orbitals. Using this information, they were able to instantly determine the sizes of particles in the liquid.

The scientists evaluated the apparatus using polystyrene nanoparticles with sizes ranging from 200 to 900 nm. These dimensions are beyond the capabilities of dynamic light scattering.

The group speculates that OF2i could be modified to measure additional nanoparticle properties, such as their shape and chemical composition.

Future work by the researchers will focus on testing whether OF2i will work for materials other than polystyrene, which is still a matter of debate at the moment. However, they hope that if Simi and colleagues' method continues to work for additional nanomaterials, it will offer a versatile bench for processing nanomaterials, opening the door to new developments in a variety of applications.

Source: PhysicsWorld

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