Talks

Drying Dynamics and Pattern Formation of Droplets

When droplets contain nonvolatile solutes, solvent evaporation can induce complex deposition patterns on substrates or form intricate particles in a vacuum. For polymer solution droplets, we developed a coarse-grained polymer-solvent droplet model to study the particle’s formation of drying polymer solution droplets. By inducing highly entangled polymers and maintaining temperatures below the glass transition point, highly porous particles are generated followed by the cavitation of solvents. Moreover, this model is extended to study the fast evaporative cooling of droplets in a vacuum. We found that a smaller droplet has a lower crystallization temperature, allowing them to remain in a liquid state even below the homogeneous freezing point. For colloidal solution droplets, our work focused on the theoretical study of their drying dynamics and deposition patterns on substrates. Using Onsager Variational Principle, we investigated various drying droplets systems. By incorporating the contact angle hysteresis mechanism, we successfully explained the formation of multi-ring deposition patterns. Additionally, introducing surfactants into the droplet allows us to control the transition between rings-shape deposition patterns. We further extended the theory to binary droplets, where the Marangoni effect results in complex spreading dynamics on a superwetting substrate. There are still an amount of interesting and complex physical and chemical phenomena within droplets, and further exploration of these phenomena promises to provide new insights and applications in multiple research fields.

IMPRS Seminar: Pattern formation by turbulent cascades

Fully developed turbulence is a universal and scale-invariant chaotic state characterized by an energy cascade from large to small scales at which the cascade is eventually arrested by dissipation. Here we show how to harness these seemingly structureless turbulent cascades to generate patterns. Pattern formation entails a process of wavelength selection, which can usually be traced to the linear instability of a homogeneous state. By contrast, the mechanism we propose here is fully nonlinear. It is triggered by the non-dissipative arrest of turbulent cascades: energy piles up at an intermediate scale, which is neither the system size nor the smallest scales at which energy is usually dissipated. Using a combination of theory and large-scale simulations, we show that the tunable wavelength of these cascade-induced patterns can be set by a non-dissipative transport coefficient called odd viscosity, ubiquitous in chiral fluids ranging from bioactive to quantum systems. Odd viscosity, which acts as a scale-dependent Coriolis-like force, leads to a two-dimensionalization of the flow at small scales, in contrast with rotating fluids in which a two-dimensionalization occurs at large scales. Apart from odd viscosity fluids, we discuss how cascade-induced patterns can arise in natural systems, including atmospheric flows, stellar plasma such as the solar wind, or the pulverization and coagulation of objects or droplets in which mass rather than energy cascades.

IMPRS Seminar: Modelling odd transport

In materials that break chiral symmetry, the stress response to applied strain can contain unusual coefficients dubbed odd viscosity (in fluids) or odd elasticity (in solids), both of which can be seen as special cases of a general phenomenon of odd viscoelasticity. In the first part of the talk, I will describe the first known microscopic model that produces an odd viscoelastic fluid. After coarse-graining the model, we analytically calculate the odd viscoelastic coefficients and corroborate the findings using molecular dynamics simulations. In the second part of the talk, I will introduce a 50-year old paradox present in the non-relativistic kinetic theory, according to which the rotation of the observer can induce odd transport properties in a non-rotating fluid. I will then show how the paradox can be resolved by phrasing the non-relativistic kinetic theory in the language of Newton-Cartan geometry, which is obtained from the relativistic Lorentzian geometry in the limit of the speed of light going to infinity.

Colloquium

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Colloquium

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IMPRS Seminar

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IMPRS Seminar

IMPRS Seminar

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