Thermodynamics of Active Matter


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The hallmark of active matter is the autonomous directed motion of its microscopic constituents driven by consumption of energy resources. This motion leads to the emergence of large-scale dynamics and structures without any equilibrium equivalent. Though active field theories offer a useful hydrodynamic description, it is unclear how to properly quantify the energetic cost of the dynamics from such a coarse-grained description. We have provided a framework to embed active field theories in a thermodynamically consistent setting, thus identifying the energy exchanges between active systems and their surrounding thermostat at the hydrodynamic level. Our current framework is based on linear irreversible thermodynamics and is thus applicable for small-scale systems that are relatively close to equilibrium. It is yet to be determined if similar framework can be developed further away from equilibrium. This approach leads to evaluating the rate of heat dissipated in the thermostat as a measure of the cost to sustain the system away from equilibrium, which is related to the `informatic’ entropy production rate that measures the irreversibility of the active field dynamics. Using this framework we can study optimal protocols for inducing externally a phase transition in active field theories and design active engines that exploit emergent order of active materials.




Odd Viscosity in Active Matter


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In common fluids, viscosity is associated with dissipation. However, when time-reversal symmetry is broken a new type of nondissipative “viscosity” emerges. Recent theories and experiments on classical 2D systems with active spinning particles have heightened interest in “odd viscosity". Recently, we have presented a microscopic Hamiltonian theory for odd viscosity in both 2D and 3D using the Poisson-bracket approach, and confirming that the OV coefficient is related to the internal angular momentum of the fluid molecules. The mere appearance of an internal angular momentum breaks isotropy, leading to the understanding that (unlike quantum fluids) in classical active materials, odd viscosity should also be present in 3D. Following this, we have found that odd viscosity should appear in various active matter realizations, specifically in active gels, where motors (such as myosin) exert mechanical torques on filaments, and bacterial suspensions in which the bacterium body rotates in the opposite direction of its flagella. Exploring some of the consequences of (constant) odd viscosity in 3D fluids reveals remarkable phenomena such as the breakdown of Bernoulli’s principle and propagation of bulk displacement waves. Our work opens a whole new line of research in which we study possible other origins of odd viscosity including spin-spin interactions and the effect of thermal fluctuations, the nature of the 3D surface states and their topological character and the possibility of new type of phase-transitions referred to as non-reciprocal phase transistion.




Active Gels


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Biological gels appear both in the extracellular matrix and in the cytoskeleton that is crucial for cell mechanics (e.g. cell division). Activity in the former originate in cells, while in the latter molecular motors such as myosin pulls/walk on the polar filaments. Many biological systems, such as bacterial suspensions and actomyosin networks, form polar liquid crystals. We introduced a novel shear-elongation parameter that capture changes in the magnitude of the polar order parameter under flow. This can give rise to a shear-induced first-order phase transition from an isotropic to a polar phase, and significantly changes the rheological properties of both active and passive polar fluids. Specifically, exotic phenomena such as nonmonotonic apparent viscosities, negative yield stress and negative viscosity were observed in active polar liquid crystals. The negative viscosity regime corresponds to active work done by the constitutes of the fluid on the boundaries, thus, revealing a method of extracting work from active polar particles. In many cases the source of activity is chiral; since forcing is internally generated, some sort of `torque dipole' is then present locally. For example, in bacteria such as E. coli, the flagellar bundle tends to rotate the fluid anticlockwise, whereas the body tends to rotate the fluid clockwise. We have proposed a general theory to account for chiral activity in polar fluids. Importantly, we discovered that there is no unique hydrodynamic description for such a fluid in the presence of torque dipoles of a given strength. Instead, at least three different hydrodynamic descriptions emerge, depending on whether we decompose each torque dipole as two point torques, two force pairs, or one point torque and one force pair where point torques create internal angular momenta of the chiral bodies (spin), whereas force pairs impact center of mass motion that contributes to fluid velocity. Remarkably, we find a direct contribution of chiral activity to the equation of motion for the polar order parameter. Hence, unlike achiral activity (e.g. force dipole), chiral activity survives even in `dry' (without momentum conservation) active systems where the fluid velocity is set to zero. We study the microscopic origins of the shear-elongation parameter as well as the consequences of the various microscopic active chiral components on the pattern formation in both `dry' and `wet' polar liquid crystals.




Charge Regulation


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Charge colloidal particles do not usually conform to the simple and popular idea that they can be characterized either as insulators with fixed surface charges or conductors with constant surface potential. In fact, when two colloidal particles with ionizable surface groups (immersed in an aqueous electrolyte solution) are brought together, both their surface charge density and surface electrostatic potential change with the particle (surface) inter-distance. This ubiquitous phenomenon stems from the dissociation/association of surface ionizable groups and is referred to as charge regulation (CR). It was elegantly formalized within the Poisson-Boltzmann theory of electrostatic interactions by Ninham and Parsegian in the 1970s. Although widely used in literatue, some conceptual aspects of CR have not been addressed. We have shown that CR macromolecule is fundamentally different from an insulator or conductor and disproved the common paradigm of CR being a generalization of these two common cases. In the standard framework of CR processes, the dissociable moieties are confined to fixed bounding macromolecular surfaces. We extended the current treatment and formulated a collective description for many mobile CR macromolecules finding notable consequences such as non-monotonic screening length and buffering.




Ionic Specific Effects: Surface tension and bulk properties of electrolyte solutions


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Ionic-specific effects date back to the late 19th century when Hofmeister measured the amount of protein precipitation from solution in presence of various salts and found a universal series of ionic activity. The same Hofmeister series emerges in a large variety of experiments in chemical and biological systems. Among others, they include forces between mica and silica surfaces, and the surface tension at the air/water and oil/water interfaces. This over century old Hofmeister puzzel is still not completely understood. The classical work of Onsager and Samaras uses the image-charge interaction to calculate the surface tension of an electrolyte solution. We have proposed a phenomenological self consistent approach that unites the Onsager-Samaras result with the ionic specificity of the Hofmeister series. Our analytical results gives further predictions for the microscopic ionic profiles close to the air/water and oil/water interfaces. We also study the effects of ion specificity, including the possible formation of Bjerrum pairs, on bulk properties of electrolyte solutions such as the effective dielectric constant.