Physics simulations with novel computing architectures: Optically bound swarms and light-driven micro-machines

这个博士项目的目的是利用计算机模拟来探索大量形状胶体粒子的光学捕获和结合。此外，该复杂系统将被用作研究处理器架构对基于物理的模拟计算效率的影响的试验台。该项目将是ARM控股有限公司和布里斯托尔大学物理学院之间的一个新的合作项目。物理学院对光阱的研究已有十多年的历史。有了紧密聚焦的激光束，就有可能操纵微米大小的粒子来施加力并组装复杂的纳米结构。一个相关的现象被称为光结合，其中光的相互散射导致微观粒子相互吸引。虽然该领域的大多数研究都涉及球形粒子，但布里斯托尔小组最近的工作以纳米线[1]为例研究了这种现象。从这项工作中可以清楚地看出，对称性的减少为方向和位置排序以及光驱动的平移和旋转提供了机会。利用光实现粒子自组织的潜力是令人兴奋的，并指出了一系列可能的应用，包括光约束群、自组织超材料、超灵敏力传感器和光驱动微机器。这个博士项目的目的是模拟该领域的一系列可能性，主要有两条主线：(1)研究对称性对光学束缚纳米结构的构型和运动的影响，(2)在考虑大量粒子时寻找紧急现象-一种形式的光驱动活性物质。模拟将需要计算流体动力学和光物质相互作用；两者都是计算要求高，涉及一系列数值技术，包括朗热万动力学，乔列斯基和LU分解，快速傅立叶变换和克雷洛夫子空间优化。与ARM的合作将使我们能够使用该系统作为测试平台，探索不同计算机架构对基于物理的模拟性能的影响。除了使用新的基于ARM的GW4超级计算机Isambard外，该项目还将使用硬件模拟器，由ARM提供，以允许评估不同处理器架构的效果，包括例如超宽矢量化，可重复的长字长浮点累加和可变核数。数值方法的优化最终将反馈到ARM数学库中。在博士项目期间，学生将在布里斯托尔大学进行大部分研究，但预计他们将在剑桥的ARM度过两个3个月的时间，接近第一年和第二年的学习结束，接受使用ARM模拟器的培训，并了解更多关于高性能计算的ARM设计理念。这个博士项目涉及EPSRC的一些战略主题。在物理领域，它支持“光物质相互作用与光学现象”主题和“生物物理与软物质物理”主题。设计光驱动机器的机会与“机器人”主题有关，而计算机体系结构的测试和优化则支持EPSRC的“微电子设计”主题。[10]张建军，张建军，张建军，等。纳米材料学报，2017,34(4):387 - 398。

The aim of this PhD project will be to use computer simulations to explore the optical trapping and binding of large numbers of shaped colloidal particles. Further, this complex system will be used as a test-bed for investigating the influence of processor architecture on computational efficiency in physics-based simulations. The project will be a new collaboration between ARM Holdings plc and the School of Physics, University of Bristol. Optical trapping has been studied in the School of Physics for more than 10 years. With tightly-focused laser beams, it is possible to manipulate micron-sized particles to apply forces and assemble complex nanostructures. A related phenomenon is known as optical binding, in which mutual scattering of light causes microscopic particles to attract each other. While most research in this area involves spherical particles, recent work from the Bristol group examines the phenomenon in the case of nanowires [1]. It is clear from this work that the reduction in symmetry affords opportunities for orientational and positional ordering, as well as light-driven translations and rotations.The potential for self-organisation of particles using light alone is exciting, and points to a range of possible applications, including optically-bound swarms, self-organised metamaterials, ultra-sensitive force sensors and light-driven micromachines. The aim of this PhD project will be to simulate a range of possibilities within this field, taking two main lines: (1) to study the influence of symmetry on the configurations and motions of optically bound nanostructures, and (2) to search for emergent phenomena when large numbers of particles are considered - a form of optically driven active matter. Simulations will require hydrodynamic and light-matter interactions to be calculated; both are computationally demanding and involve a range of numerical techniques, including Langevin dynamics, Cholesky and LU decompositions, fast Fourier transforms and Krylov sub-space optimisations. The collaboration with ARM will enable us to use this system as a test-bed for exploring the influence of different computer architectures on the performance of physics-based simulations. As well as using the new GW4 ARM-based supercomputer, Isambard, the project will employ hardware emulators, courtesy of ARM, to allow the effects of different processor architectures to be assessed, including e.g. super-wide vectorisation, reproducible long wordlength floating point accumulation and variable numbers of cores. Optimisations of the numerical methods will ultimately be fed back into the ARM maths libraries.During the PhD project, the student will perform most of their research at the University of Bristol, but it is anticipated that they will spend two 3-month periods at ARM in Cambridge, towards the ends of the first and second years of study, to receive training in the use of the ARM emulators, and to learn more about the ARM design philosophy for high performance computing.This PhD project relates to a number of EPSRC strategic themes. In the physics realm, it supports both the "Light matter interaction and optical phenomena" theme and the "Biophysics and soft matter physics" theme. The opportunity to design light-driven machines relates to the "Robotics" theme, while the testing and optimisation of computer architecture supports EPSRC's "Microelectronics design" theme. [1] Simpson, S.H. et al., Nano Letters, 17, 3485-3492 (2017).