Manipulation of deformable particles in optical and flow fields

Optical tweezers are ubiquitous in many fields of physics, with applications ranging from single molecule biophysics to nano-assembly of complex structures and machines. In optical tweezers, focussed laser beams are used to apply and measure forces in the pico- to nano-Newton range. The magnitude and direction of the optical forces generated depend on the laser beam shape and particle geometry, and several techniques have been developed in recent years for evaluating these.
Non-conservative motion is an area of interest in optical tweezers, which arises from the fact that there is a flow of optical momentum through the system and trapping arises from a balance of dynamical forces. This has led to recent observations of novel behaviour such as the optical wing [1]. The coupling of optical tweezers with microfluidic devices opens up a range of novel possibilities; the combination of fluid and optical momentum should herald many new applications including novel force transduction mechanisms, optical sorting techniques and optically-driven micro-machines.
The introduction of deformable particles introduces further degrees of freedom, and hence of control, to the system. Microfluidic devices are often used in lab-on-a-chip type applications involving the transport of cells in a fluid. Red blood cells, for example, are relatively flexible and deform as they are transported through blood vessels and other narrow channels. They may also be deformed with optical tweezers, and this may form the basis of a medical diagnostic device [2]. Particles which deform while being optically trapped have not been studied analytically, except in the special case of the optical stretcher [3]. The combination of shaped optical fields with deformable trappable particles in a flow field opens up intriguing possibilities of new applications, as well as providing opportunities to study fundamental aspects of the light-matter interaction, including the well-known Abraham-Minkowski controversy [4].
The aim of this PhD project will be to develop computational models for calculating the trapping behaviour of irregular and deformable particles in flow fields, and apply these to the trapping of cells, micelles and more rigid non-spherical structures in flow fields. It will build on previous work in the group, in particular combining use of the discrete dipole approximation for light-matter interactions [5] with Brownian dynamics simulations to capture the hydrodynamic behaviour. The project will exploit the latest computing technology available as part of the Bristol University super-computer, BlueCrystal. There will be opportunities both to collaborate with experi-mentalists, and to undertake experimental work, to explore the predictions of the models.
[1] Swartzlander Jr., GA, et al., Nature Photonics 5: 48-51 (2011).
[2] Agrawal R, et al., Scientific Reports 6: 15873 (2016).
[3] Guck, J, et al., Biophys. J., 81: 767-784 (2001).
[4] Pfeifer, RNC, et al., Reviews of Modern Physics. 79: 1197-1216 (2007).
[5] Simpson, SH, et al., Optics Express, 19:16526-16541 (2011).