Wetting and Spreading with Soft Materials

非技术摘要：流体的润湿和扩散出现在整个自然和工程系统中。虽然液滴在刚性基板上的动力学是由分离三（固/液/气）相的接触线控制的，但软材料提出了新的挑战。对于柔顺的基材，接触线上的力会引起下垫表面的变形，进而影响液滴的润湿和扩散。这是一个高度非线性的问题，可以导致大范围的行为。预测和控制它们与医疗技术、微流体流动、组织生长和细菌生物膜有关。当涉及的材料不仅是柔软的，而且是有生命的，扩散甚至可以由自流化驱动，增加了复杂性。了解这一过程可以为有效地流化工业颗粒流或在干发酵过程中引入循环提供新的手段。该项目不仅资助本科生和研究生进行研究，而且通过将流变仪和表面张力仪放置在物理系的共享教育和研究实验室，为该系的其他学生提供材料表征方面的新培训。一年一度的“开启你的物理之旅！”(跳跃!)工作坊鼓励来自不同背景的高中女生通过实践工作坊来探索物理。研究小组努力招募和支持传统上代表性不足的群体的学生，既从事实验室研究，又参加传播/推广活动。技术摘要：本研究项目解决了关于小尺度相互作用如何引起软质材料中液滴润湿和扩散的宏观效应的开放性问题。研究小组利用两种模型系统来理解变形和流动之间复杂的相互作用。第一个系统解决了凝胶/液体/气体界面的接触线力，使用系统变化的材料参数和几何形状来解决软基板上毛细力和弹性力的相互作用。第二个系统使用了一种新的模式生物，酪螨（干酪螨）菌落，为研究生物水动力流动的发生和动力学开辟了新的途径。这项工作从凝胶和活性流体的详细特征开始，测量诸如复杂剪切模量、泊松比、表面张力和活性度等量。了解这些材料参数将允许团队设计实验，探测参数值的必要范围，也允许模型的比较和开发。总之，这两个系统允许团队回答以下问题：目前围绕静态接触线模型的争议是否可以通过一系列材料的实验来合理化？接触线的曲率是否起着额外的作用？通过重力和/或材料特性的梯度，可以实现哪些新的液滴扩散和传输模式？在有生命的流体中，粒子尺度的位置波动是否可以被认为类似于普通流体中的热波动？

Non-technical abstract: The wetting and spreading of fluids appears throughout natural and engineered systems. While the dynamics of droplets resting on rigid substrates are well-understood to be controlled by the contact line separating the three (solid/liquid/gas) phases, soft materials present new challenges. For compliant substrates, the forces at the contact line cause a deformation of the underlying surface, which in turn impacts the wetting and spreading of the droplet. This is a highly nonlinear problem that can result in a large range of behaviors. Predicting and controlling them is relevant to medical technologies, microfluidic flows, tissue growth, and bacterial biofilms. When the materials involved are not just soft, but living, the spreading can even be driven by self-fluidization, increasing the degree of complexity. Understanding this process could provide new means for efficiently fluidizing industrial granular flows, or for introducing circulation into dry fermentation processes. The project not only funds undergraduate and graduate students to perform research, but makes new training in materials characterization available to other students in the department through the rheometer and surface tensiometer to be placed in the physics department's shared Education and Research Lab. The annual "Launch your Excellent Adventures with Physics!" (LEAP!) Workshop encourages high school girls from diverse backgrounds to explore physics through hands-on workshops. The research team strives to recruit and support students who are members of traditionally under-represented groups, both to work on laboratory research and to take part in dissemination/outreach activities. Technical abstract: This research project addresses open questions about how small-scale interactions give rise to macroscopic effects in the wetting and spreading of droplets in soft materials. The research team utilizes two model systems to understand the complex interplay between deformation and flow. The first system addresses the contact line forces at a gel/liquid/gas interface, using systematically-varied material parameters and geometries to resolve the interaction of capillary and elastic forces on soft substrates. The second system uses a new model organism, colonies of Tyrolichus casei (cheese mites), to open a new avenue of study into the onset and dynamics of living hydrodynamic flows. The work starts from detailed characterizations of both the gels and the living fluids to measure such quantities as the complex shear modulus, Poisson ratio, surface tension, and degree of activity. Knowledge of these material parameters will allow the team to both design experiments which probe the necessary range of parameter values, and also allow for comparison and development of models. Together, these two systems allow the team to answer the following questions: Can current controversies surrounding static contact line models be rationalized through experiments on a range of materials? Does the curvature of the contact line play an additional role? What new modes of spreading and transport of droplets can be achieved through gravity and/or gradients in materials properties? Can particle-scale position fluctuations in living fluids be thought of as analogous to the thermal fluctuations in an ordinary fluid?