Precise characterization of micro rotors in optical tweezers

We present an optical tweezer based study of rotation of microscopic objects with shape asymmetry. Thermal fluctuations and rotations are simultaneously monitored through laser back scattering. The rotation results in a modulation in intensity of the back scattered light incident on a quadrant photo detector. This results in the manifestation of peaks at a fundamental rotational frequency and at integer harmonics, superimposed on a modified Lorentzian in the power spectrum. The multiple peaks indicate that the rotations are periodic but with varying angular velocity. We demonstrate the use of video microscopy for characterization of low reflectivity rotors, such as biological cells. The methods also enable a measurement of the average torque on the rotor, and in principle, can reveal information about its principal moments of inertia, and the role of hydrodynamics at micron levelsComment: 9 Pages, 6 Figure

Characterizing the rotation of non symmetric objects in an optical tweezer

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

An optical tweezer-based study of antimicrobial activity of silver nanoparticles

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. © Indian Academy of Sciences

A Dual Optical Tweezer for Microrheology of Bacterial Suspensions

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 soln. Using this setup, we report measurements of the microrheol. parameters of Pseudomonas fluorescens and Bacillus subtilis bacterial suspensions. We study the variation of viscoelastic moduli of these bacterial suspensions as a function of their cell count in soln. A comparison with inactive bacteria of corresponding cell count enables us to characterize the activity of the bacterial samples in terms of an av. force that the bacteria exerts on the trapped bead. This work paves way for studies of interesting nonlinear rheol. phenomena at small length scales

Probing the interaction between two microspheres in a single Gaussian beam optical trap

Interactions between trapped microspheres have been studied in two geometries so far: (i) using line optical tweezers and (ii) in traps using two counter propagating laser beams. In both trap geometries, the stable inter bead separations have been attributed to optical binding. One could also trap two such beads in a single beam Gaussian laser trap. While there are reports that address this configuration through theoretical or simulation based treatments, there has so far been no detailed experimental work that measures the interactions. In this work, we have recorded simultaneously the fluctuation spectra of two beads trapped along the laser propagation direction in a single Gaussian beam trap by measuring the back scattered signal from the trapping and a tracking laser beam that are counter propagating . The backscattering from the trapping laser monitors the bead encountered earlier in the propagation path. The counter propagating tracking laser, on the other hand, is used to monitor the fluctuations of the second bead. Detection is by using quadrant photo detectors placed at either end. The autocorrelation functions of both beads reveal marked departures from that obtained when there is only one bead in the trap. Moreover, the fall-off profiles of the autocorrelation indicates the presence of more than one relaxation time. This indicates a method of detecting the presence of a second bead in a trap without directly carrying out measurements on it. Further, a careful analysis of the relaxation times could also reveal the nature of interactions between the beads. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Microrheology of non mulberry silk varieties by optical tweezer and video microscopy based techniqueas

We have carried out a comparative study of the microrheol. properties of silk fibroin solns. formed from a variety of silks indigenous to the Indian subcontinent. We present the measured viscoelastic moduli of Tasar silk fibroin soln. using both a single and dual optical tweezer at 0.16​% and 0.25​% (w​/v)​. The bandwidth of the measurements carried out using optical tweezers is extended down to the lower frequency regime by a video microscopy measurement. Further, we have measured the viscoelastic moduli of Eri and Muga varieties of silk fibroin solns. at a higher concn. (1.00​% w​/v) limiting the tool of measurement to video microscopy, as the reduced optical transparencies of these solns. at higher concn. preclude an optical tweezer based investigation. The choice of a higher concn. of fibroin soln. of the latter silk varieties is so as to enable a comparison of the shear moduli obtained from optical methods with their corresponding fiber stiffness obtained from wide angle X-​ray scattering data. We report a correlation between the microstructure and microrheol. parameters of these silk varieties for the concn. of fibroin solns. studied

Experimental Investigation on Mechanical Properties of Hemp/E-Glass Fabric Reinforced Polyester Hybrid Composites

This research work has been focusing on Hemp fibers has an alternative reinforcement for fiber reinforced polymer composites due to its eco-friendly and biodegradable characteristics. This work has been carried out to evaluate the mechanical properties of hemp/E-glass fabrics reinforced polyester hybrid composites. Vacuum bagging method was used for the preparation of six different kinds of hemp/glass fabrics reinforced polyester composite laminates as per layering sequences. The tensile, flexural, impact and water absorption tests of these hybrid composites were carried out experimentally according to ASTM standards. It reveals that an addition of E-glass fabrics with hemp fabrics can increase the mechanical properties of composites and decrease the water absorption of the hybrid composites

A Helix Replacement Mechanism Directs Metavinculin Functions

Cells require distinct adhesion complexes to form contacts with their neighbors or the extracellular matrix, and vinculin links these complexes to the actin cytoskeleton. Metavinculin, an isoform of vinculin that harbors a unique 68-residue insert in its tail domain, has distinct actin bundling and oligomerization properties and plays essential roles in muscle development and homeostasis. Moreover, patients with sporadic or familial mutations in the metavinculin-specific insert invariably develop fatal cardiomyopathies. Here we report the high resolution crystal structure of the metavinculin tail domain, as well as the crystal structures of full-length human native metavinculin (1,134 residues) and of the full-length cardiomyopathy-associated ΔLeu954 metavinculin deletion mutant. These structures reveal that an α-helix (H1′) and extended coil of the metavinculin insert replace α-helix H1 and its preceding extended coil found in the N-terminal region of the vinculin tail domain to form a new five-helix bundle tail domain. Further, biochemical analyses demonstrate that this helix replacement directs the distinct actin bundling and oligomerization properties of metavinculin. Finally, the cardiomyopathy associated ΔLeu954 and Arg975Trp metavinculin mutants reside on the replaced extended coil and the H1′ α-helix, respectively. Thus, a helix replacement mechanism directs metavinculin's unique functions