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

Rheological behaviour of native silk feedstocks

Whilst much is known about the properties of silks, the means by which native silk feedstocks are spun still represent a gap in our knowledge. Rheology of the native silk feedstocks is germane to an understanding of the natural spinning process. Yet, an overview of the literature reveals subtle limitations and inconsistencies between studies, which has been largely attributed to sample-to-sample variation when testing these exquisitely flow-sensitive materials. This ambiguity has prevented reliable, consistent inferences from standard polymer rheology and constitutes an obstacle to further development. To address this challenge, we present the largest study to date into the rheological properties of native silk feedstocks from Bombyx mori larvae. A combination of shear and oscillatory measurements were used to examine in detail the relationships between concentration, low shear viscosity, relaxation times, complex modulus and estimates of the molecular weights between entanglements. The results from this highly detailed survey will provide a sound basis for further experimental or theoretical work and lay the foundations for future bio-inspired processing of proteins

Investigations on rheological properties and gelation of tasar regenerated silk fibroin solution

Tasar silk is a variety of non-mulberry silk indigenous to the Indian subcontinent. We present the measured frequency-dependent viscoelastic moduli of Tasar regenerated silk fibroin (RSF) solution using optical tweezers at two concentrations (0.16 and 0.25 w/v) and extend these measurements to the low frequency regime using a video microscopy technique. We extend the investigation on the rheological behavior of Tasar RSF for four more RSF concentrations, viz., 0.50, 1.00, 2.50 and 5.00 using video microscopy. In all the RSF samples, both storage and loss moduli are found to increase with frequency. At lower frequencies the loss modulus is more than the storage modulus and exhibit similar behavior until a crossover frequency beyond which the storage modulus exceeds the loss modulus at all frequencies. The relaxation time which is inversely related to the crossover frequency is found to rise sharply at 5 w/v, indicating the onset of gelation in the sample. These results are examined in relation to the viscoelastic parameters of mulberry silk, wherein the larger crossover frequencies at the same higher concentrations indicate relaxation times that are an order of magnitude smaller than those measured for Tasar RSF. © 2013 Wiley Periodicals, Inc

Orientational dynamics of human red blood cells in an optical trap

We report here on studies of reorientation of human red blood cells (RBCs) in an optical trap. We have measured the time required, tre, for the plane of the RBC entering the optical trap to undergo a 90-deg rotation to acquire an edge on orientation with respect to the beam direction. This has been studied as a function of laser power, P, at the trap center. The variation of tre with increasing P shows an initial sharp decrease followed by a much smaller rate of further decrease. We find that this experimentally measured variation is not in complete agreement with the variation predicted by a theoretical model where the RBC is treated as a perfectly rigid circular disk-like body. We argue that this deviation arises due to deformation of the RBC. We further reason that this feature is dominated by the elastic behavior of the RBC membrane. We compare the studies carried out on normal RBCs with RBCs where varying conditions of membrane stiffness are expected. We propose that the value of energy used for maximum deformation possible during a reorientation process is an indicator of the membrane elasticity of the system under study. © 2013 The Authors

Optical properties of red blood cells: An optical tweezer based analysis

A microscopic object finds an equilibrium orientation under a laser tweezer such that a maximum of its volume lies in the region of highest electric field. Furthermore, birefringent microscopic objects show no rotational diffusion after reorienting under a linearly polarized optical trap and also are seen to follow the plane of polarization when the latter is changed using a half wave plate. We observe that a healthy human Red Blood Cell (RBC) reproduces these observations in an optical tweezer, which confirms it to be birefringent. Polarization microscopy based measurements reveal that the birefringence is confined to the cell's dimple region and the mean value of retardation for polarized green light (λ = 546nm) is 9 ± 1.5nm. We provide a simple geometrical model that attributes the birefringence to the nature of arrangement of the phospholipid molecules of the bilayer. This predicts the observed variation in the measured birefringence, from the dimple to the rim of the cell which we further show, can serve to demarcate the extent of the dimple region. This points to the value of birefringence measurements in revealing cell membrane contours.. We extend this technique to understand the birefringence of a chicken RBC, an oblate shaped cell, wherein the slow axis is identified to be coincident with the long axis of the cell. Further, we observe the birefringence to be confined to the edges of the cell. Experiments to probe the optomechanical response of the chicken RBC are in progress. © 2014 SPIE

Root Anatomical Traits and Their Possible Contribution to Drought Tolerance in Grain Legumes

Legumes are mostly grown rainfed and are exposed to various types of drought ranging from terminal drought to intermittent drought. The objective of this study was to compare the root anatomy of six major legume crops in relation to their drought adaptation strategies. Plants of chickpea (Cicer arietinum L.), groundnut (Arachis hypogaea L.), pigeonpea (Cajanus cajan [L.] Millsp.), cowpea (Vigna unguiculata L. walp.), soybean (Glycine max [L.] Merr.) and common bean (Phaseolus vulgaris L.) were grown along with pearl millet (Pennisetum glaucum [L.] R. Br.) in a Vertisol field during the rainy season of 2010. Four root segments from 35-day-old plants of each crop species were collected, 10 cm from the root tip and used for making transverse sections. These root segments were thinner in both groundnut and pigeonpea than in other legumes but similar to those of pearl millet. Soybean and pigeonpea had a relatively thinner cortex than the other legumes. Xylem vessel size and the numbers were apparently the most discriminating traits of legumes. Pigeonpea is equipped to conduct small quantities of water per unit time with a few narrow xylem vessels and that explains the conservative early growth of pigeonpea. Chickpea and cowpea showed moderate xylem passage per root indicating that they are capable of absorbing water moderately and are well equipped for regular drought episodes. The development of cortical and stele tissue and their proportion is markedly influenced by moisture availability to the root system