Rotational dynamics of Bacillus subtilis in an optical trap

Ashwini V Bhat, Praveen Parthasarathi, Shruthi S Iyengar, Balaji Yendeti, D C Mohana, Ashok Vudayagiri and Sharath Ananthamurthy

The swimming of a bacterium in fluids occurs in a low Reynolds number regime. The ability to confine the swimming motion by trapping a bacterium in laser light, can give information on the propulsion coefficients, which are important in explaining the efficiency of swimming of these bacteria. In this work, we report the results of an optically trapped Bacillus subtilis in an optical tweezer and the studies on the rotatory motion of the bacterium. The data is gathered and analysed using video microscopy. The propulsion coefficients of such swimming bacterium are determined through a power spectral analysis of the rotatory motion of the bacterium in the trap.

DOI

The mechanical oscillation of a single carbon nanocoil driven by a focused laser beam

Y L Liu, J Shen and Y M Sun

We have determined the mechanical properties of a long single carbon nanocoil (ALS-CNC) interacting with a focused laser beam. The mechanical oscillation properties of ALS-CNC in liquid based on the interaction of a CNC with light have been demonstrated. Considering the viscous force of a liquid, ALS-CNC oscillation does not occur by laser irradiation when ALS-CNC is in air. A CNC switch controlled by laser irradiation was realized. The first section in your paper.

DOI

Studying effect of carrier fluid viscosity in magnetite based ferrofluids using optical tweezers

S Savitha, Shruthi S Iyengar, Sharath Ananthamurthy and Sarbari Bhattacharya

Ferrofluids with varying viscosities of carrier fluids have been prepared with magnetite (Fe3O4) nanoparticles. The nanoparticles were synthesized by chemical co-precipitation and characterized using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). They were found to be nearly spherical in shape with an almost uniform size of 13nm. The superparamagnetic nature of the water based ferrofluids at room temperature was established by SQUID magnetometry. Dynamic light scattering (DLS) was carried out to establish the size of the nanoparticle clusters in the ferrofluids synthesized. The results indicate an increase in cluster size with increase in carrier fluid viscosity. This is supported by results from Raman Spectroscopy. A further attempt to characterise these ferrofluids was made by studying the behaviour of well characterised non-magnetic micron sized probes that are optically trapped while suspended in the ferrofluid. An increase in carrier fluid viscosity results in a decrease in corner frequency when only the carrier fluid is used as the suspending medium. When the magnetic component is also present the corner frequency is higher than with just the carrier fluid. This relative increase happens at all laser powers at the trapping plane. This trend is also found to be independent of the size and material of the probe particle. Comparisons of various parameters that influence optical trapping lead us to believe that the enhancement could be due to a directed motion of the magnetic clusters in the presence of an optical trap.

DOI