RHEOLOGY, ENTROPY PRODUCTION AND RATCHETING OF DEFORMABLE ACTIVE SYSTEMS

NON-TECHNICAL SUMMARY This award supports theoretical research and education that aims at understanding three important aspects of active materials that are currently unexplored. Active matter is an exciting new field in materials engineering and has come into prominence over the last decade. This occurred because of the development of micro-particles capable of self-propulsion. One can think of these particles as synthetic analogs of bacteria whose shape, surface chemistry and velocity can be designed in a lab. There are several fundamental questions that need to be addressed in this field and that offer new opportunities for the development of the next generation of smart materials. What is the elastic response to external stimuli of materials formed by active particles? This is an important problem that can improve our understanding of the mechanical properties of a large number of materials, from biofilms to epithelial tissue. Another unexplored issue concerns the behavior of active surfaces. Unlike synthetic vesicles, the motion of a biological cell is completely determined by complex biochemical reactions. This makes their behavior similar to that of active systems. It is therefore important to understand how surfaces/vesicles respond to active forces. For instance, fibroblasts and epithelial cancerous cells can acquire directional motion when confined within rigid, asymmetric micro-channels. The PI will explore to what extent such a behavior can be captured by a much simpler system: a synthetic vesicle loaded with active particles. The goal is to develop simple synthetic analogs of biological cells capable of mimicking their mechanical behavior. Finally, one of the main characteristics of active systems is that they break time-reversal symmetry, i.e. running time backwards on a particle trajectory does take the particle back along the same path from which it came. The degree to which this happens in a system can be quantified by measuring Entropy Production. The team will measure this quantity for a number of active systems and establish a link between spatial gradients in Entropy Production and the degree to which active systems are out of equilibrium and capable of performing work. The outcomes of this research will provide insight into how to design stimuli-responsive materials capable of performing work at the microscale. The numerical tools developed for this project should be transferable to other active systems and will have important implications for a number of biological problems that rely on similar physical mechanisms. The award contributes to the education of undergraduate and graduate students which the PI will recruit to participate in these projects, and they will have first-hand exposure to cutting-edge numerical and statistical methods to model active systems. Furthermore, an outreach plan in collaboration with a number of on-campus organizations such as WISC (Women in Science at Columbia), whose efforts are dedicated to the advancement of women and underrepresented minorities in the sciences, technology, engineering and math, is currently underway, and will be further extended.TECHNICAL SUMMARYThis award supports theoretical research and education that aims at understanding three important issues of active materials that are currently unexplored. The first issue concerns the rheological properties of active condensates. Although a large body of work has been devoted to studying self-assembly, dynamics and the phase behavior of active colloidal particles, limited work has been done to understand the elastic properties of active condensates and their response to external stimuli. This is a fundamental problem that needs to be addressed to better characterize the mechanical properties of these materials. The second issue concerns the interplay between elastic and active forces on fluid vesicles using models that allow for topological transitions. The PI will explore under what conditions rectification can occur when giant unilamellar vesicles are loaded with active particles. While a good amount of work has been done to understand motion rectification of single active particles, not much is known about the transport properties of soft, deformable interfaces activated by self-propelling particles across micro-channels. This is an important problem given that fibroblasts and epithelial cancerous cells can acquire directional motion when confined within asymmetric, periodic channels, and would present a minimal model for the description of such a complex system. The third issue deals with a fundamental question about the very nature of active systems. Although the most intriguing phenomenological behavior of active systems arises, at the most fundamental level, because of time-reversal symmetry breaking, and entropy production is the hallmark signature of lack of equilibrium, a clear relationship between inhomogeneities in local entropy production and the degree to which active systems are out of equilibrium and capable to perform work has not been adequately established. The PI will explore how knowledge of spatial gradients in entropy production can be exploited to maximize the work active systems can perform at the microscale. The outcomes of this research will advance our current knowledge of statistical physics and will provide insight into how to design stimuli-responsive materials capable of performing work at the microscale. The numerical tools developed for this project should be transferable to other active systems and will have important implications for a number of biological problems that rely on similar physical mechanisms. The award contributes to the education of undergraduate and graduate students which the PI will recruit to participate in these projects, and they will have first-hand exposure to cutting-edge numerical and statistical methods to model active systems. Furthermore, an outreach plan in collaboration with a number of on-campus organizations such as WISC (Women in Science at Columbia), whose efforts are dedicated to the advancement of women and underrepresented minorities in the sciences, technology, engineering and math, is currently underway, and will be further extended.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.