Director alignment at the nematic–isotropic interface: elastic anisotropy and active anchoring

Activity in nematics drives interfacial flows that lead to preferential alignment that is tangential or planar for extensile systems (pushers) and perpendicular or homeotropic for contractile ones (pullers). This alignment is known as active anchoring and has been reported for a number of systems and described using active nematic hydrodynamic theories. The latter are based on the one-elastic constant approximation, i.e. they assume elastic isotropy of the underlying passive nematic. Real nematics, however, have different elastic constants, which lead to interfacial anchoring. In this paper, we consider elastic anisotropy in multiphase and multicomponent hydrodynamic models of active nematics and investigate the competition between the interfacial alignment driven by the elastic anisotropy of the passive nematic and the active anchoring. We start by considering systems with translational invariance to analyse the alignment at flat interfaces and, then, consider two-dimensional systems and active nematic droplets. We investigate the competition of the two types of anchoring over a wide range of the other parameters that characterize the system. The results of the simulations reveal that the active anchoring dominates except at very low activities, when the interfaces are static. In addition, we found that the elastic anisotropy does not affect the dynamics but changes the active length that becomes anisotropic. This article is part of the theme issue ‘Progress in mesoscale methods for fluid dynamics simulation’.

Nanofluids for the Next Generation Thermal Management of Electronics: A Review

Nowadays, the thermal management of electronic components, devices and systems is one of the most important challenges of this technological field. The ever-increasing miniaturization also entails the pressing need for the dissipation of higher power energy under the form of heat per unit of surface area by the cooling systems. The current work briefly describes the use on those cooling systems of the novel heat transfer fluids named nanofluids. Although not intensively applied in our daily use of electronic devices and appliances, the nanofluids have merited an in-depth research and investigative focus, with several recently published papers on the subject. The development of this cooling approach should give a sustained foothold to go on to further studies and developments on continuous miniaturization, together with more energy-efficient cooling systems and devices. Indeed, the superior thermophysical properties of the nanofluids, which are highlighted in this review, make those innovative fluids very promising for the aforementioned purpose. Moreover, the present work intends to contribute to the knowledge of the nanofluids and its most prominent results from the typical nanoparticles/base fluid mixtures used and combined in technical and functional solutions, based on fluid-surface interfacial flows.

Coupling Vortical Bulk Flows to the Air–Water Interface: From Putting Oil on Troubled Waters to Surfactants on Protein Solutions

The air–water interface in flowing systems remains a challenge to model, even in cases where the interface is essentially flat. This is because even though each side is governed by the Navier–Stokes equations, the stress balance which provides the boundary conditions for the equations involves properties associated with surfactants that are inevitably present at the air–water interface. Aside from challenges in measuring interfacial properties, either intrinsic or flow-dependent, the two-way coupling of bulk and interfacial flows is non-trivial, even for very simple flow geometries. Here, we present an overview of the physics associated with surfactant monolayers of flowing liquid and describe how the monolayer affects the bulk flow and how the monolayer is transported and deformed by the bulk flow. The emphasis is primarily on cylindrical flow geometries, and both Newtonian and non-Newtonian interfacial responses are considered. We consider interfacial flows that are solenoidal as well as those where the surface velocity is not divergence free.

Numerical simulation of water film edge dynamics

<p>Fundamental contribution to the formation of sea spray under strong winds is provided by the bag-breakup phenomenon - rupture of film in the form of parachute [1]. Breaking of water film into droplets is caused, among other factors, by processes occurring on the free edge of the film moving under action of surface tension forces. The study of these processes will help to understand how characteristics of the film and the drops appearing after its rupture are related.</p><p>Using the Basilisk software package with Volume of Fluid advection scheme for interfacial flows, numerical simulation of three-dimensional water film placed in domain filled with air was carried out. The water film was placed into domain filled with air. One of the edges of the film is free, and the second is fixed on the left boundary of the domain; along the third coordinate, the boundary conditions are periodic. At the initial moment of time, the film is defined by a sheet with variable thickness - the upper boundary has the form of a cosine. The change in the shape of the film over time was recorded. It is revealed that the inhomogeneity of the film thickness leads to the appearance of a significant curvature of the edge of the film as it moves under the action of surface tension forces.</p><p>This work was supported by the RFBR grants (20-05-00322, 21-55-50005, 21-55-52005) and RSF grant 19-17-00209.</p><p>[1] Troitskaya, Y. et al. Bag-breakup fragmentation as the dominant mechanism of sea-spray production in high winds. Sci. Rep. 7, 1614 (2017).</p>

Control and Optimization of Interfacial Flows Using Adjoint-Based Techniques

The applicability of adjoint-based gradient computation is investigated in the context of interfacial flows. Emphasis is set on the approximation of the transport of a characteristic function in a potential flow by means of an algebraic volume-of-fluid method. A class of optimisation problems with tracking-type functionals is proposed. Continuous (differentiate-then-discretize) and discrete (discretize-then-differentiate) adjoint-based gradient computations are formulated and compared in a one-dimensional configuration, the latter being ultimately used to perform optimisation in two dimensions. The gradient is used in truncated Newton and steepest descent optimisers, and the algorithms are shown to recover optimal solutions. These validations raise a number of open questions, which are finally discussed with directions for future work.