Wednesday, 23 September 2015

SPIE OTOM XII Conference Proceedings

The proceedings of the SPIE Optical Trapping and Optical Manipulation Conference XII, held in San Diego in August 2015 have been published, including three contributions from the UCL Optical Tweezers Group:

T. J. Smart, C. J. Richards, R. Bhatnagar, C. Pavesio, R. Agrawal and P. H. Jones.  'A study of red blood cell deformability in diabetic retinopathy using optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945820, doi 10.1117/12.2191281 (2015)
From the abstract: Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus (DM) in which high blood sugar levels cause swelling, leaking and occlusions in the blood vessels of the retina, often resulting in a loss of sight. The microvascular system requires red blood cells (RBCs) to undergo significant cellular deformation in order to pass through vessels whose diameters are significantly smaller than their own. There is evidence to suggest that DM impairs the deformability of RBCs, and this loss of deformability has been associated with diabetic kidney disease (or nephropathy) - another microvascular complication of DM. However, it remains unclear whether reduced deformability of RBCs correlates with the presence of DR.

Here we present an investigation into the deformability of RBCs in patients with diabetic retinopathy using optical tweezers. To extract a value for the deformability of RBCs we use a dual-trap optical tweezers set-up to stretch individual RBCs. RBCs are trapped directly (i.e. without micro-bead handles), so rotate to assume a `side-on' orientation. Video microscopy is used to record the deformation events, and shape analysis software is used to determine parameters such as initial and maximum RBC length, allowing us to calculate the deformability for each RBC.  A small decrease in deformability of diabetes cells subject to this stretching protocol is observed when compared to control cells.

T. J. Smart, C. J. Richards, Xiang Han, S. Siwiak-Jaszek and P. H. Jones.  'Correlated fluctuations of optically trapped particles',  Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945823, doi 10.1117/12.2190820 (2015)
From the abstract: We present a study of correlated Brownian fluctuations between optically confined particles in a number of different configurations.  First we study colloidal particles held in separate optical tweezers.  In this configuration the particles are known to interact through their hydrodynamic coupling, leading to a pronounced anti-correlation in their position fluctuations at short times.  We study this system and the behavior of the correlated motion when the trapped particles are subject to an external force such as viscous drag.

The second system considered is a chain of optically bound particles in an evanescent wave surface trap.  In this configuration the particles interact both through hydrodynamic and optical coupling.  Using digital video microscopy and subsequent particle tracking analysis we study the thermal motion of the chain and map the covariance of position fluctuations between pairs of particles in the chain.  The experiments are complemented by Brownian motion simulations. 

C. J. Richards, T. J. Smart, P. H. Jones and D. Cubero.  'Low frequency dynamical stabilisation in optical tweezers', Proc SPIE 9548, Trapping and Optical Micromanipulation XII, 945825, doi 10.1117/12.2190822 (2015)
From the abstract: It is well known that a rigid pendulum with minimal friction will occupy a stable equilibrium position vertically upwards when its suspension point is oscillated at high frequency.  The phenomenon of the inverted pendulum was explained by Kapitza by invoking a separation of timescales between the high frequency modulation and the much lower frequency pendulum motion, resulting in an effective potential with a minimum in the inverted position.

We present here a study of a microscopic optical analogue of Kapitza's pendulum that operates in different regimes of both friction and driving frequency.  The pendulum is realized using a microscopic particle held in a scanning optical tweezers and subject to a viscous drag force.  The motion of the optical pendulum is recorded and analyzed by digital video microscopy and particle tracking to extract the trajectory and stable orientation of the particle.  In these experiments we enter the regime of low driving frequency, where the period of driving is comparable to the characteristic relaxation time of the radial motion of the pendulum with finite stiffness.

In this regime we find stabilization of the pendulum at angles other than the vertical (downwards) is possible for modulation amplitudes exceeding a threshold value where, unlike the truly high frequency case studied previously, both the threshold amplitude and equilibrium position are found to be functions of friction.  Experimental results are complemented by an analytical theory for induced stability in the low frequency driving regime with friction.

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