Seminar

Optical tweezers are scientific instruments capable of manipulating nano and micro scale particles by exerting very small forces of the order of piconewtons via a highly focused laser beam. The forces exerted on a bead suspended in a medium due to thermal fluctuations can also be measured using optical tweezers. Optical trapping of a single nanoparticle in a Gaussian intensity profile single beam optical tweezer may not always be possible except by deliberate design. However, the dynamics of nanoparticles present in a fluid medium can be detected through changes in the trapping characteristics of an optically trapped microsphere present in the same fluid suspension as that of the nanoparticles. The trapped microsphere which serves as a microprobe is seen to be sensitive to physical and chemical alterations to the suspension medium which affect the overall dynamics of the nanoparticles.

In the case where the nanoparticles themselves respond to external magnetic fields, the combination of an optically trapped non-magnetic microprobe with the surrounding ferrofluid, can detect the influence of magnetic micro entities in the suspension on the ferrofluid particles and vice versa. The analysis of the trapping characteristics can also reveal the dynamics of such magnetic micro and nano entities under external magnetic field gradients.

Replacing the non-magnetic microprobe with a core shell structured magnetic microprobe, while adding to the promise of enhanced magnetic field detection capacities, also sets up viscosity and refractive index gradients in the surrounding medium due to localized heating in the vicinity of the probe. Combination probes of optically trapped microbeads and suspensions of nanoparticles can thus serve to characterize the effect of physical and chemical changes as well as magnetic and thermal inhomogeneities in the local environment.

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