A normal human red blood cell (RBC) when trapped with a linearly polarized laser, reorients about the electric polarization direction and then remains rotationally bound to this direction. This behavior is expected for a birefringent object. We have measured the birefringence of distortion-free RBCs in an isotonic medium using a polarizing microscope. The birefringence is confined to the cell's dimple region and the slow axis is along a diameter. We report an average retardation of 3.5 ± 1.5 nm for linearly polarized green light (λ = 546 nm). We also estimate a retardation of 1.87 ± 0.09 nm from the optomechanical response of the RBC in an optical trap. We reason that the birefringence is a property of the cell membrane and propose a simple model attributing the origin of birefringence to the phospholipid molecules in the lipid bilayer and the variation to the membrane curvature. We observe that RBCs reconstituted in shape subsequent to crenation show diminished birefringence along with a sluggish optomechanical response in a trap. As the arrangement of phospholipid molecules in the cell membrane is disrupted on crenation, this lends credence to our conjecture on the origin of birefringence. Dependence of the birefringence on membrane contours is further illustrated through studies on chicken RBCs.

A dual optical tweezer has been built around an inverted microscope with high numerical aperture objective (N.A 1.4). The setup is versatile and can be used both as a single and a dual tweezer, and in the dual mode, enables us to optically trap two micron-sized latex beads within a few microns from each other in solution. Using this setup, we report measurements of the microrheological parameters of Pseudomonas°uorescens and Bacillus subtilis bacterial suspensions. We study the variation of viscoelastic moduli of these bacterial suspensions as a function of their cell count in solution. A comparison with inactive bacteria of corresponding cell count enables us to characterize the activity of the bacterial samples in terms of an average force that the bacteria exerts on the trapped bead. This work paves way for studies of interesting nonlinear rheological phenomena at small length scales.

We have studied in detail the situation wherein two microbeads are trapped axially in a single-beam Gaussian intensity profile optical trap. We find that the corner frequency extracted from a power spectral density analysis of intensity fluctuations recorded on a quadrant photodetector (QPD) is dependent on the detection scheme. Using forward- and backscattering detection schemes with single and two laser wavelengths along with computer simulations, we conclude that fluctuations detected in backscattering bear true position information of the bead encountered first in the beam propagation direction. Forward scattering, on the other hand, carries position information of both beads with substantial contribution from the bead encountered first along the beam propagation direction. Mie scattering analysis further reveals that the interference term from the scattering of the two beads contributes significantly to the signal, precluding the ability to resolve the positions of the individual beads in forward scattering. In QPD-based detection schemes, detection through backscattering, thereby, is imperative to track the true displacements of axially trapped microbeads for possible studies on light-mediated interbead interactions.

Abstract. Understanding and characterizing microbial activity reduction in the presence of antimicrobial agents can help in the design and manufacture of antimicrobial drugs. We demonstrate the use of an optical tweezer setup in recording the changes in bacterial activity with time, induced by the presence of foreign bodies in a bacterial suspension. This is achieved by monitoring the fluctuations of an optically trapped polystyrene bead immersed in it. Examining the changes in the fluctuation pattern of the bead with time provides an accurate characterization of the reduction in the microbial activity. Here, we report on the effect of addition of silver nanoparticles on bacterial cultures of Pseudomonas aeroginosa, Escherichia coli and Bacillus subtilis. We observe a decrease in the bacterial activity with time for the investigated bacterial samples. This method in our opinion, enables one to track changes in bacterial activity levels as a function of time of contact with the antibacterial agent with greater efficacy than traditional cell counting methods.

We present an optical tweezer based study of the rotation of microscopic objects with shape asymmetry. Thermal fluctuations and rotations are simultaneously monitored through laser back scattering. The rotation causes a modulation in intensity of the back scattered light incident on a quadrant photo detector. The resulting power spectrum is a modified Lorentzian with additional peaks located at the fundamental rotational frequency of the object and at the integer harmonics. The manifestation of these peaks reveals that the rotations are periodic but with varying angular velocity. We model our experimental results to illustrate the hydrodynamic interplay between the rotor and the surrounding medium that results in the time dependence of the angular speed of the former. Further, we demonstrate the use of video microscopy for characterization of low reflectivity rotors, such as biological cells. We propose through these studies that an analysis of these rotations can provide insights into the role of hydrodynamics at micron levels.