Self-assembly and motility far from equilibrium

技术总结材料研究部和分子和细胞生物科学部为该奖项提供资金。该奖项支持理论和计算研究和教育，以了解生物有机体推动自身的物理机制。某些能动的生物对象，如单核细胞增生性李斯特菌和不对称细菌中的复制染色体，如新月弯杆菌和霍乱弧菌，通过聚合或解聚蛋白细丝来产生它们的推进力。这在蛋白质中建立了浓度梯度，因此物体一侧比另一侧有更多的细丝。保持周围小溶质不对称浓度的胶体将以确定的速度推动自己穿过流体。这种现象被称为自扩散渗透，已经被用作微型或纳米游泳者的一种推进手段。在一个典型的例子中，浓度梯度由胶体上催化化学反应的活性区域控制，导致活性区域附近比胶体远侧产生更多的产物和更少的反应物。溶质在胶体周围的分布由扩散和平流共同决定。胶体和溶质之间的相互作用建立了最终推动颗粒的流体流动。如果溶质很小，所以它的扩散比平流快，我们知道只需要两个条件就能实现运动：可移动的物体必须能够在稳定状态下保持不对称的溶质分布，并且溶质和物体之间必须有净相互作用。该奖项支持理论和计算研究，旨在了解与李斯特菌、弧菌和弧菌相关的新制度，其中粒子表面催化自组装或分解，而不是涉及简单离子或分子的化学反应。有待解决的问题包括：自我扩散渗透的机制能否解释已经在实验中研究过的生物扰动的影响？当溶质很大并且具有有效地消失的扩散常数时，它足以产生不对称的溶质浓度分布吗，就像在生物例子中一样？在平流型和扩散主导型中，自扩散潜水作用有何不同？由细丝的自组装和分解驱动的运动是否比由简单溶质的化学反应驱动的运动更能抵抗反作用力？该项目将训练物理学和生物学界面上的学生结合生物物理学和远离平衡的物理，这两个领域在NRC CMMP-2010研究中被确定为凝聚态和材料物理学的重大挑战。学生将接受跨学科建模和计算方法方面的培训，为他们进入劳动力市场提供多才多艺的能力。非技术性摘要材料研究部和分子与细胞生物科学部为该奖项提供资金。该奖项支持理论和计算研究，旨在了解某些活着的有机体--特别是细菌或细菌内的成分(如染色体)--是如何推动自己前进的。这种生物生活在流动的环境中，所以它们必须游泳。像李斯特菌这样的单核细胞增多症细菌会在它们的后部产生分支细丝，推动它们向前移动，这样它们就可以感染其他细胞，而霍乱弧菌中的染色体会分解它们前面的蛋白质细丝，以便在细胞分裂之前在细胞中移动。众所周知，微米大小的颗粒在一侧涂覆有诸如铂的化学反应启用材料，可以通过在表面实现化学反应来推动自己通过流体。在这种情况下，产品或反应物与颗粒表面之间的相互作用会推动流体流动，最终推动颗粒前进。但细丝的组装和拆卸对流体流动的影响与简单的化学反应和流体流动的相互作用有很大的不同。本研究的目的是了解细丝的组装或分解可以驱动运动的物理机制，并探索这种运动是否比材料界已经研究的简单的反应驱动的运动对物理和生化扰动更强大。纤维组装驱动的推进是生物体中许多免疫过程的关键，包括免疫细胞如何移动、癌细胞如何扩散以及细胞在伤口愈合期间如何迁移。更好地了解这些细胞如何运动的物理机制，对于开发帮助或阻碍它们的方法可能很重要。这些生物实现已经进化得非常强大，也可能激发出更好的运动材料，如微型或纳米游泳者，它们可以对抗强大的相反力量向前移动，并可能对包括药物输送在内的应用有用。该奖项将通过培训研究生在物理科学和生命科学的交叉点上建立模型和计算方法，为他们在工作中迅速演变的挑战做好准备。接触不同的领域，并有机会直接与生物学家以及理论、计算和实验物理学家合作，将使他们准备好与拥有非常不同专业领域的同事进行有效的合作和沟通。

Technical SummaryThe Division of Materials Research and the Division of Molecular and Cellular Biosciences contribute funds to this award. This award supports theoretical and computational research and education to understand the physical mechanism by which living organisms propel themselves. Certain motile biological objects, such as the bacterium Listeria monocytogenes and replicating chromosomes in asymmetric bacteria like Caulobacter crescentus and Vibrio cholerae, generate their propulsion by polymerizing or depolymerizing protein filaments. This establishes a concentration gradient in the protein so that there are more filaments on one side of the object than the other. A colloid that maintains an asymmetric concentration of a small solute around it will propel itself through a fluid with a well-defined velocity. This phenomenon, known as self-diffusiophoresis, has been exploited as a means of propulsion of micro- or nano-swimmers. In a typical example, the concentration gradient is controlled by an active region on the colloid that catalyzes a chemical reaction, leading to more product and less reactant near the active region than on the far side of the colloid. The distribution of solute around the colloid is determined by a combination of diffusion and advection. An interaction between the colloid and solute sets up fluid flow that ultimately propels the particle. If the solute is small, so that its diffusion is rapid compared to advection, it is known that only two conditions are needed to achieve motility: the motile object must be able to maintain an asymmetric solute distribution in steady state, and there must be a net interaction between the solute and the object.This award supports theoretical and computational research aimed at understanding a new regime relevant to Listeria, Caulobacter and Vibrio, in which the particle surface catalyzes self-assembly or disassembly rather than a chemical reaction involving simple ions or molecules. Questions to be addressed include: Can the mechanism of self-diffusiophoresis explain effects of biological perturbations that have been studied experimentally? Does it suffice to produce an asymmetric concentration profile of a solute when the solute is large and has an effectively vanishing diffusion constant, as in the biological examples? How does self-diffusiophoresis differ in the advection and diffusion-dominated regimes? Might motility driven by self-assembly and disassembly of filaments be more robust to opposing forces than motility driven by chemical reactions involving simple solutes?The project will train students at the interface of physics and biology to combine biophysics and far-from-equilibrium physics, two areas that have been identified as grand challenges for condensed matter and materials physics in the NRC CMMP-2010 study. Students will be trained both in interdisciplinary model-building and in computational methods, providing versatility for their entry into the workforce.Nontechnical SummaryThe Division of Materials Research and the Division of Molecular and Cellular Biosciences contribute funds to this award. This award supports theoretical and computational research designed to understand how certain living organisms - particularly bacteria or components within bacteria such as chromosomes - propel themselves. Such organisms live in a fluid environment so they must swim. Bacteria such as Listeria monocytogenes, responsible for listeriosis, generate branched filaments at their rear that propel them forward so that they can infect other cells, while chromosomes in Vibrio cholera, responsible for cholera, disassemble protein filaments in front of them in order to move across the cell before the cell divides. It is known that micron-sized particles that are coated on one side with a chemical reaction enabling material such as platinum can propel themselves through a fluid by enabling a chemical reaction at the surface. In that case, an interaction between the product or reactant and the particle surface drives fluid flow that ultimately propels the particles forward. But the interplay of filament assembly and disassembly on fluid flow is very different from that of simple chemical reactions with fluid flow. The aim of this research is to understand the physical mechanism by which the assembly or disassembly of filaments can drive motion, and to explore whether such locomotion might be more robust to physical and biochemical perturbations than the simple reaction-driven locomotion that has been studied in the materials community.Filament-assembly-driven propulsion is key to many immune processes in living organisms, including how immune cells move, how cancer cells spread and how cells migrate during wound healing. A better understanding of the physical mechanism underlying how these cells move may be important to developing ways of helping or hindering them. These biological realizations, which have evolved to be remarkably robust, may also inspire better motile materials such as micro- or nano-swimmers that can move forwards against large opposing forces and may be useful for applications including drug delivery.This award will prepare graduate students for rapidly evolving challenges in the workforce by training them in model-building and computational methods at the intersection of the physical and life sciences. The exposure to different fields and opportunity to work directly with biologists as well as theoretical, computational and experimental physicists will prepare them to collaborate and communicate effectively with colleagues with very different areas of expertise.