Session J15: Nano Flows: Computations and Modeling (8:00am - 8:45am CST)

Directed Percolation in Demixing Blends on Wetting Substrate

Demixing, also called phase separation, is a challenging scientific problem with many important applications. When liquid-liquid mixtures demix in contact with a surface, often, one of the liquids accumulates next to the surface which drastically alters the resulting morphologies. The dynamical evolution of these demixing morphologies is crucial for the performance, amongst others, of many optoelectronic and photovoltaic devices because their functioning relies on a well-defined morphology that must have (i) a minimum local concentration in each phase, and (ii) connectivity to the substrate in order to transport positive and negative charges to the corresponding electrodes. We investigate demixing in binary mixtures on a wetting substrate from the perspective of directed and connectivity percolation. Our results provide an improved understanding of the onset and growth of percolation, that may assist to tailor-make morphologies as per application.   

Molecular Dynamics Study of Janus Particle Aggregates in Shear Flows

Janus nanoparticles have attracted much interest recently because of the way that they interact with each other to self-assemble into complex nanostructures. Theoretical studies indicate that amphiphilic Janus nanoparticles experience a torque in fluids due to their slip-asymmetric boundary conditions, and our molecular dynamics simulations have verified this [1]. As a result, Janus self-assembled nanostructures could be unstable in fluid flows. Most previous molecular dynamics studies have used soft-sphere potentials to study Janus nanoparticle self-assembly processes, but such methods don’t capture the effect of slip boundary conditions at the spheres’ surfaces. We have studied thermal- and shear-induced break-up of Janus and homogeneous hydrophobic dimers in a fluid using a hard-sphere potential [2]. We will present our latest results investigating the possible mechanisms which lead to increased break-up probability, and propose a theoretical expression for all effective parameters contributing to the breakup rate. Overall, the Janus dimers are less stable than hydrophobic dimers, and their stability depends on the slip length at the spheres’ surfaces. [1] Safaei, Sina, et al. Soft Matter (2019). [2] Safaei, Sina, Shaun C. Hendy, and Geoff R. Willmott. Soft Matter (2020).   

Thermodynamic analysis of multivalent binding of functionalized nanoparticles to the cell membrane surface

h $-abstract-$\backslash $pardWe present a quantitative model for multivalent binding of ligand-coated flexible polymeric nanoparticles (NPs) to a membrane expressing receptors. The model is developed using a multiscale computational framework by coupling a continuum field model for the cell membrane with a coarse-grained model for the NP. The NP is modeled as a bead-spring polymer chain, and the membrane is modeled as a dynamically triangulated surface. The NP binding affinity to a cell surface is mainly determined by the delicate balance between the enthalpic gain due to the ligand-receptor binding and the entropic penalties of various components. We show that the multivalent interactions between the NP and the cell surface are subject to entropy-enthalpy compensation. Three different entropy contributions, namely, those due to receptor-ligand translation, NP flexibility, and membrane undulations, are all significant, although the first of these terms is the most dominant. However, both NP flexibility and membrane undulations dictate the receptor-ligand translational entropy making the entropy compensation context-specific, i.e., dependent on whether the NP is rigid or flexible, and on the state of the membrane. $\backslash $pard-/abstract-$\backslash $\tex   

Nanoscale Fin Effects in Heat Transfer at the Fluid-Solid Interface

Nanoscale heat transfer at a fluid-solid interface can exhibit surprising behaviors that have no macroscopic analog. In this talk, we present results from molecular-dynamics (MD) simulations in which we study the heat-transfer performance of surfaces patterned with nanoscale fins, in systems held at a fixed thermal gradient. Such problems are relevant for understanding thermal transport properties of interfaces featuring nanoscale patterns or nanomaterial coatings. In MD simulations of a simple fluid, we find that nanoscale fins can induce significant fluid structuring effects as well as anomalous fluid diffusion at the fluid-solid interface. We show that the magnitudes of these effects can be accurately captured using recently developed models. We also present results on the heat transfer coefficient in the system as a function of the thermal gradient, the fluid density, and the fin geometry (size and aspect ratio); these results differ in several ways from the results of classical fin theory, in part due to fluid layering effects. We rationalize these results by examining the vibrational density of states in the vicinity of the fluid-solid interface.   

Structure and Transport Properties of Liquid Crystals under Nanoscale Confinement

Significantly confined films of liquid crystals appear in a wide variety of technologies. The structure and transport properties of nanoconfined liquid crystals differ significantly from a bulk (unconfined) phase. In this work, we present theoretical and computational results on liquid crystals confined within slits whose sizes are ones to tens of the mesogen lengthscale. To study these systems, we perform molecular-dynamics (MD) simulations of fluids consisting of ellipsoidal particles, of a variety of aspect ratios, whose interactions are given by the Gay-Berne interatomic potential. While systematically varying the fluid density and temperature, we compute a number of fluid properties, including density, diffusivity, and statistics quantifying orientational order. Our results demonstrate the importance of adsorption effects induced by the solid boundaries, which significantly affect both the structure and transport, especially in the near-wall region, as well as the location of an isotropic-to-nematic phase transition for the system overall. We present an analytical model based on a mean-field theory, which enables us to characterize density inhomogeneities under confinement; the predictions of this model are validated by comparison with MD simulations   

Influence of In-Plane Liquid Ordering on Thermal Resistance Boundary Effects at Liquid/Solid Interfaces

Applications ranging from control of small scale avionics to AI computing platforms are becoming ever more reliant on efficient cooling of high power 3D integrated chips prone to hot spots and thermal runaway. Thermal extraction is now the limiting factor in information processing. As a result, convective cooling using fans and gases is being replaced by liquid cooled microfluidic channels which take advantage of the higher heat capacity of liquids. The decrease in channel dimension size also increases the importance of boundary effects. Here we rely on molecular dynamics simulations to examine the magnitude of the thermal jump in quiescent systems known to occur at liquid/solid interfaces. Most previous studies have examined the influence of liquid density stratification near the solid wall, wettability effects and wall symmetry on the magnitude of the thermal jump. Here we explore in-plane ordering of the first liquid layer adjacent to the solid wall by tuning the local temperature, thermal flux and parameters controlling the intermolecular potentials. Our results, some intuitive and some not, yield a surprisingly general correlation for the thermal jump which reflects the collective response of the first liquid layer.   

Continuum Models of Nanoscale Effects in Initial Stages of Droplet Coalescence and Wetting

In molecular dynamics (MD) simulations of nanodrop coalescence [1] and wetting two non-classical effects are observed. First, despite the axial symmetry of the problems of impact and head-on drop-drop collisions, initial contact generally occurs away from the symmetry axis due to thermal fluctuations of the droplet surface. Second, in initial stages of coalescence/wetting, growth of the liquid bridge/contact area mostly occurs via transverse jumps of the liquid molecules across the gap, rather than via lateral motion expected classically. For the first effect, we consider stochastic differential equations for the modes of surface deformation to obtain results for the distribution of contact points, in good agreement with MD. For the second effect, we solve numerically Navier-Stokes equations with a disjoining pressure derived from the Lennard-Jones potential used in MD and thus having both an attractive and a repulsive term. Attraction causes the droplet surface to accelerate towards the surface it is coming in contact with and reach a significant speed by the time of contact, which contributes to the motion of the contact line, as in MD. [1] S. Perumanath \textit{et al.}, Phys. Rev. Lett. 122 (2019) 104501.