Laser-induced assembly of biological cells and colloids onto a candle soot coated substrate

Recently, laser-induced heating assisted assembly of colloidal particles is emerging as a popular technique against direct light-assisted particle assembling methods. Herein, we demonstrate the trapping and assembly of yeast cells and upconversion particles at laser excitation intensities as low as 11 μW/μm² via localized heating of non-plasmonic candle soot coated biocompatible polymer, polydimethylsiloxane. The thermal convective flow around the irradiation zone plays a vital role in trapping and assembling of the objects. The flow velocity profile is symmetrical around the irradiation zone, and the velocity increases drastically near the irradiation zone and with laser intensity. An increase in laser intensity leads to formation of an air bubble at the liquid-substrate interface and subsequent assembly of the objects at the interface. The tunneling of objects towards the irradiation zone across the barrier is demonstrated. Finally, with the localized laser-induced heating, the surface charge independent swarm of upconversion particle is shown, and the size and range of which found to be laser intensity dependent. With the wavelength-independent low power operation in the near-infrared range through simple optics, the results presented here may open avenues for a wide range of applications in diverse areas, including life sciences, colloidal science, nanoscience, photonics, and material science.