Manipulation of deformable particles in optical and flow fields

光学镊子在物理学的许多领域中无处不在，应用范围从单分子生物物理到复杂结构和机器的纳米组装。在光学镊子中，聚焦的激光光束被用来施加和测量皮牛顿到纳米牛顿范围内的力。产生的光学力的大小和方向取决于激光光束的形状和粒子的几何形状，近年来已经开发了几种技术来评估这些技术。非保守运动是光镊子中的一个重要领域，它产生于这样一个事实，即光动量流过系统，而俘获产生于动态力的平衡。这导致了最近观察到的新行为，如光学翅膀[1]。光学镊子与微流控器件的结合开辟了一系列新的可能性；流体和光学动量的结合将预示着许多新的应用，包括新奇的力传递机制、光学分选技术和光学驱动微机械。可变形粒子的引入为系统带来了更多的自由度，从而引入了控制。微流控装置通常用于芯片实验室类型的应用，涉及流体中细胞的传输。例如，红细胞在通过血管和其他狭窄通道运输时相对灵活和变形。它们也可以用光学镊子变形，这可能构成医疗诊断设备的基础[2]。除了光学拉伸器的特殊情况外，还没有对光学捕获时变形的粒子进行解析研究[3]。成形光场与可变形粒子在流场中的结合为新的应用开辟了有趣的可能性，也为研究光-物质相互作用的基本方面提供了机会，包括著名的亚伯拉罕-明可夫斯基争论[4]。该PHD项目的目的是开发计算模型来计算不规则和可变形粒子在流场中的捕获行为，并将这些模型应用于在流场中捕获细胞、胶束和更坚硬的非球形结构。它将建立在该小组以前的工作基础上，特别是结合使用光-物质相互作用的离散偶极近似[5]和布朗动力学模拟来捕捉流体动力学行为。该项目将利用布里斯托尔大学超级计算机BlueCrystal可用的最新计算技术。将有机会与实验者合作，并进行实验工作，以探索模型的预测。[1]Swartzlander Jr.，GA等人，《自然光子学》5：48-51(2011年)。[2]Agrawal R等人，科学报告6：15873(2016年)。[3]Guck，J等人，生物物理学。J.81：767-784(2001).[4]Pfeifer，RNC等人，现代物理学评论.[5]辛普森，SH等人，《光学快报》，19：16526-16541(2011年)。

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).