Dynamics of liquid nanojets

Molecular dynamics of a water jet from a carbon nanotube

A carbon nanotube (CNT) can be viewed as a molecular nozzle. It has a cylindrical shape of atomistic regularity, and the diameter can be even less than 1 nm. We have conducted molecular-dynamics simulations of water jet from a (6,6) CNT that confines water in a form of single-file molecular chain. The results show that the water forms nanoscale clusters at the outlet and they are released intermittently. The jet breakup is dominated by the thermal fluctuations, which leads to the strong dependence on the temperature. The cluster size n decreases and the release frequency f increases at higher temperatures. The f roughly follows the reaction kinetics by the transition state theory. The speed of a cluster is proportional to the 1/sqrt[n] because of the central limit theorem. These properties make great contrast with the macroscopic liquid jets.

Simulations of liquid nanocylinder breakup with dissipative particle dynamics

In this work, we use a dissipative-particle-dynamics-based model for two-phase flows to simulate the breakup of liquid nanocylinders. Rayleigh's criterion for capillary breakup of inviscid liquid cylinders is shown to apply for the cases considered, in agreement with prior molecular dynamics (MD) simulations. Also, as shown previously through MD simulations, satellite drops are not observed, because of the dominant role played by thermal fluctuations which lead to a symmetric breakup of the neck joining the two main drops. The parameters varied in this study are the domain size, cylinder radius, thermal length scale, viscosity, and surface tension. The breakup time does not show the same scaling dependence as in capillary breakup of liquid cylinders at the macroscale. The time variation of the radius at the point of breakup agrees with prior theoretical predictions from expressions derived with the assumption that thermal fluctuations lead to breakup.

Thermal noise influences fluid flow in thin films during spinodal dewetting

Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Because of thermal fluctuations the spectrum changes from an exponential to a power law decay for large wave vectors. Also the time evolution of the typical wave vector of unstable perturbations exhibits noise-induced coarsening that is absent in deterministic hydrodynamic flow.

Numerical methods for the stochastic Landau-Lifshitz Navier-Stokes equations

The Landau-Lifshitz Navier-Stokes (LLNS) equations incorporate thermal fluctuations into macroscopic hydrodynamics by using stochastic fluxes. This paper examines explicit Eulerian discretizations of the full LLNS equations. Several computational fluid dynamics approaches are considered (including MacCormack's two-step Lax-Wendroff scheme and the piecewise parabolic method) and are found to give good results for the variance of momentum fluctuations. However, neither of these schemes accurately reproduces the fluctuations in energy or density. We introduce a conservative centered scheme with a third-order Runge-Kutta temporal integrator that does accurately produce fluctuations in density, energy, and momentum. A variety of numerical tests, including the random walk of a standing shock wave, are considered and results from the stochastic LLNS solver are compared with theory, when available, and with molecular simulations using a direct simulation Monte Carlo algorithm.

Universality crossover of the pinch-off shape profiles of collapsing liquid nanobridges in vacuum and gaseous environments

Liquid propane nanobridges were found through molecular dynamics simulations to exhibit in vacuum a symmetric break-up profile shaped as two cones joined in their apexes. With a surrounding gas of sufficiently high pressure, a long-thread profile develops with an asymmetric shape. The emergence of a long-thread profile, discussed previously for macroscopic fluid structures, originates from the curvature-dependent evaporation-condensation processes of the nanobridge in a surrounding gas. A modified stochastic hydrodynamic description captures the crossover between these universal break-up regimes.

Drop formation by thermal fluctuations at an ultralow surface tension

We present experimental evidence that drop breakup is caused by thermal noise in a system with a surface tension that is more than 10(6) times smaller than that of water. We observe that at very small scales classical hydrodynamics breaks down and the characteristic signatures of pinch-off due to thermal noise are observed. Surprisingly, the noise makes the drop size distribution more uniform, by suppressing the formation of satellite droplets of the smallest sizes. The crossover between deterministic hydrodynamic motion and stochastic thermally driven motion has repercussions for our understanding of small-scale hydrodynamics, important in many problems such as micro- or nanofluidics and interfacial singularities.

The interface in demixed colloid-polymer systems: wetting, waves and droplets

Phase transitions in colloid-polymer mixtures have attracted a large amount of attention over the last 20 years (W. C. K. Poon, , 2002, , R859; R. Tuinier, J. Rieger and C. G. de Kruif, , 2003, , 1). By comparison, the interfacial tension between the coexisting phases has received little attention. Here, we show that the ultralow interfacial tension in fluid-fluid demixed colloid-polymer systems, which is roughly one million times smaller than in ordinary liquids, manifests itself in a wide variety of interface characteristics and processes. Discussed are the interfacial wetting behaviour close to a hard wall, the thermal capillary waves at the free interface and the process of droplet coalescence and breakup. These subjects can be studied in a single experiment by combining modern soft matter chemistry and laser scanning confocal microscopy. This combination allows a further exploration of a broad range of interface issues.

Charge Reduction in Electrosprays: Slender Nanojets as Intermediates

Molecular dynamics simulations were used to study charge reduction in electrosprayed liquids through the formation of slender nanojet intermediates. The dynamics of shape relaxation and disintegration were followed as a function of charge in cylindrical water nanojets containing protonated diglycine molecules. Depending on the overall charge, simulations showed three basic scenarios for nanojet evolution. Moderately charged nanojets reduced to spheres, whereas nanojets charged close to the Rayleigh limit divided into two offspring droplets. Due to the large Coulomb interaction between ions, highly charged nanojets suffered repeated fission until the resulting droplets were charged below the Rayleigh limit. We demonstrated the role of surface fluctuations and Maxwell stress distributions in the disintegration process. The relaxation dynamics of the moderately charged systems to spherical geometry followed a damped oscillator behavior. Compared to neutral water jets, the presence of charges in subcritical nanojets resulted in a stiffer system with longer relaxation times to spherical geometry. Interparticle forces acting between the separating offspring droplets in nanojet breakup were also determined. Due to the increased role of fluctuations in nanojets, the Rayleigh limit was shown to overestimate the maximum charge on stable systems indicating higher nanodroplet production efficiency than one would expect from macroscopic theories alone.

Repeated formation of fluid threads in breakup of a surfactant-covered jet

Breakup of thin threads is widely observed in nature and technology. As a surfactant-covered liquid jet approaches breakup, its profile consists of a periodic pattern of drops connected by thin threads. Near the locations where the threads join the drops, simulations show that a series of thinner threads arise as the jet breaks. That threads can continue to form repeatedly without addition of noise when surfactants are present is unexpected based on earlier studies of surfactant-free systems. Thinning dynamics of successive threads are shown to be self-similar and approach Eggers's universal solution for clean interfaces.

Materials by numbers: computations as tools of discovery

Current issues pertaining to theoretical simulations of materials, with a focus on systems of nanometer-scale dimensions, are discussed. The use of atomistic simulations as high-resolution numerical experiments, enabling and guiding formulation and testing of analytic theoretical descriptions, is demonstrated through studies of the generation and breakup of nanojets, which have led to the derivation of a stochastic hydrodynamic description. Subsequently, I illustrate the use of computations and simulations as tools of discovery, with examples that include the self-organized formation of nanowires, the surprising nanocatalytic activity of small aggregates of gold that, in the bulk form, is notorious for being chemically inert, and the emergence of rotating electron molecules in two-dimensional quantum dots. I conclude with a brief discussion of some key challenges in nanomaterials simulations.

Mechanism for flow-rate controlled breakup in confined geometries: a route to monodisperse emulsions

This Letter describes a quasistationary breakup of an immiscible, inviscid fluid at low capillary numbers. The breakup proceeds in a coflowing, viscous liquid, in a confined geometry of a long and narrow orifice. In contrast to the capillary instability in an unbounded fluid, the collapse proceeds through a series of equilibria, each yielding the minimum interfacial energy of the fluid-fluid interface. The process is slow in comparison to typical relaxation speeds of the interface, and it is reversible. Its quasistatic character of collapse forms the basis for controlled, high-throughput generation of monodisperse fluid dispersions.

Experimental observation of nanojets formed by heating PbO-coated Pb clusters

We are reporting the first experimental observation of nanojets formed by heating PbO-coated Pb clusters, which has been predicted theoretically by Moseler and Landman. During heating, the hot liquid is ejected through the broken orifice into a vacuum and forms a condensed trail in the shape of a tadpole, as shown in the transmission electron micrographs. The temperature-variable Raman spectra indicate that nanojet formation is closely related to the heating temperature and thus essentially to the pressure in the coated clusters. The pressure inside the shell rises from the inner core's melting and its confined volume expansion. It then drops after the final explosion, dominating the whole nanojetting process.

Noise, coherent fluctuations, and the onset of order in an oscillated granular fluid

We study fluctuations in a vertically oscillated layer of grains below the critical acceleration for the onset of ordered standing waves. As onset is approached, transient disordered waves with a characteristic length scale emerge and increase in power and coherence. The scaling behavior and the shift in the onset of order agrees with the Swift-Hohenberg theory for convection in fluids. However, the noise in the granular system is an order of magnitude larger than the thermal noise in the most sensitive convecting fluid experiments to date; the effect of the granular noise is observable 20% below the onset of order.

Damped finite-time singularity driven by noise

We consider the combined influence of linear damping and noise on a dynamical finite-time singularity model for a single degree of freedom. We find that the noise effectively resolves the finite-time singularity and replaces it by a first-passage-time distribution or absorbing state distribution with a peak at the singularity and a long time tail. The damping introduces a characteristic cross-over time. In the early time regime the probability distribution and first-passage-time distribution show a power law behavior with scaling exponent depending on the ratio of the nonlinear coupling strength to the noise strength. In the late time regime the behavior is controlled by the damping. The study might be of relevance in the context of hydrodynamics on a nanometer scale, in material physics, and in biophysics.

Laser-induced hydrodynamic instability of fluid interfaces

We report on a new class of electromagnetically driven fluid interface instability. Using the optical radiation pressure of a cw laser to bend a very soft near-critical liquid-liquid interface, we show that it becomes unstable for sufficiently large beam power P, leading to the formation of a stationary beam-centered liquid microjet. We explore the behavior of the instability onset by tuning the interface softness with temperature and varying the size of the exciting beam. The instability mechanism is experimentally demonstrated. It simply relies on total reflection of light at the deformed interface whose condition provides the universal scaling relation for the onset P(S) of the instability.