Dynamics of liquid nano-threads : fluctuation-driven instability and rupture

The instability and rupture of nanoscale liquid threads is shown to strongly depend on thermal fluctuations. These fluctuations are naturally occurring within molecular dynamics (MD) simulations and can be incorporated via fluctuating hydrodynamics into a stochastic lubrication equation (SLE). A simple and robust numerical scheme is developed for the SLE that is validated against MD for both the initial (linear) instability and the nonlinear rupture process. Particular attention is paid to the rupture process and its statistics, where the `double-cone’ profile reported by Moseler & Landmann [Science, 2000, 289(5482): 1165-1169] is observed, as well as other distinct profile forms depending on the flow conditions. Comparison to the Eggers’ similarity solution [Physical Review Letters, 2002, 89(8): 084502], a power law of the minimum thread radius against time to rupture, shows agreement only at low surface tension; indicating that surface tension cannot generally be neglected when considering rupture dynamics

Effects of dynamic wetting and liquid-solid slip on self-propelled nanodrops in tapered nanochannels

Drops inside tapered microchannels exhibit self-propelled behavior, driven by the capillary pressure gradient within the drops. This driven force may be balanced by the viscous drag and the contact line drag to determine the drop displacement, in analogy to the way to predict capillary imbibition. However, how the drops move exactly with time at the nanoscale is unclear. This study employs molecular dynamics simulations to explore the dynamics of nanodrops within tapered channels with hydrophobic and hydrophilic coatings. The simulations reveal that in a hydrophobic tapered channel, drops migrate toward the wider side of the channel but may halt midway as the driving pressure approaches zero during their movements. Conversely, in hydrophilic tapered channels, drops move unlimitedly toward the channel's tip. Incorporating considerations for dynamic contact angles based on the molecular kinetic theory and liquid-solid slip, a theoretical model is derived that accurately predicts the drop displacement observed in molecular simulations without free parameters. In our simulations of drop motion in hydrophilic tapered channels, the drop displacement x is found linear with time x ∼ t , as the viscous drag is dominant and the slip length is small. However, the theory further predicts that drop displacement may behave as x 2 ∼ t when slip length is large. Conversely, under dominant contact line drag, the theory predicts x 3 ∼ t for drop motion in tapered nanoslits. These findings underscore the critical influence of dynamic wetting and liquid-solid slip in precisely predicting drop motions on solid surfaces at the nanoscale.</p

Slip-enhanced Rayleigh-Plateau instability of a liquid film on a fibre

Boundary conditions at a liquid-solid interface are crucial to dynamics of a liquid film coated on a fibre. Here a theoretical framework based on axisymmetric Stokes equations is developed to explore the influence of liquid-solid slip on the Rayleigh-Plateau instability of a cylindrical film on a fibre. The new model not only shows that the slip-enhanced growth rate of perturbations is overestimated by the classical lubrication model, but also indicates a slip-dependent dominant wavelength, instead of a constant value obtained by the lubrication method, which leads to larger drops formed on a more slippery fibre. The theoretical findings are validated by direct numerical simulations of Navier-Stokes equations via a volume-of-fluid method. Additionally, the slip-dependent dominant wavelengths predicted by our model agree with the experimental results provided by Haefner. et al.[Nat. Commun., Vol. 6(1), 2015, 18 pp. 1-6]

Revisiting the Rayleigh-Plateau instability for the nanoscale

The theoretical framework developed by Rayleigh and Plateau in the 19th century has been remarkably accurate in describing macroscale experiments of liquid cylinder instability. Here we re-evaluate and revise the Rayleigh-Plateau instability for the nanoscale, where molecular dynamics experiments demonstrate its inadequacy. A new framework based on the stochastic lubrication equation is developed that captures nanoscale flow features and highlights the critical role of thermal fluctuations at small scales. Remarkably, the model indicates that classically stable (i.e. ‘fat’) liquid cylinders can be broken at the nanoscale, and this is confirmed by molecular dynamics

Fluctuating hydrodynamics of nanoscale interfacial flows

Understanding the influence of thermal fluctuations on nanoscale interfacial flows is crucial to a range of modern and emerging technologies, such as in lab-on-a-chip technology and next generation 3D printing. In this thesis, effects of thermal fluctuations on two specific flows (nano-jets and bounded nano-films) are studied in detail with: (i) Molecular dynamics (MD) used as `numerical experiments'; and (ii) Landau-Lifshtz Navier-Stokes equations (LLNS, also known as fluctuating hydrodynamics equations) as an approximate, but numerically efficient, alternative. To pursue theoretical results and relatively cheap numerical solutions, further simplifications to LLNS equations, which use a long-wave approximation, are studied: (i) the stochastic lubrication equation (SLE) for nano-jets; and (ii) the stochastic thin-film equation (STFE) for bounded nano-films. The famous Rayleigh-Plateau (RP) theory is re-evaluated and revised for the instability of nanoscale jets, where MD experiments demonstrate its inadequacy. A new framework based on the SLE is developed, which captures nanoscale flow features and highlights the critical role of thermal fluctuations at small scales. Remarkably, the model indicates that classically stable (i.e. `fat') liquid cylinders can be broken at the nanoscale, and this is confirmed by MD. A simple and robust numerical scheme is then developed for the SLE, which is validated against MD for both the initial (linear) instability and the nonlinear rupture process. Particular attention is paid to the rupture process and its statistics, where the double-cone profile reported by Moseler & Landmann [1] is observed, as well as other distinct profile forms depending on the flow conditions. Comparison to the similarity solution in Eggers [2], a power law of the minimum thread radius against time to rupture, shows agreement only at low surface tension; indicating that surface tension cannot generally be neglected when considering rupture dynamics. For bounded nano-films, STFEs are developed to accommodate substrate roughness and slip boundary conditions (BCs). An efficient solver with a new iteration method, verified by the theoretical models, is then developed to explore the nonlinear dynamics of nano-droplet spreading and coalescence. Numerical solutions of the spreading denote that the slip BC accelerates the process in both the deterministic and stochastic regimes, which is supported by the power laws of the similarity solutions derived. Additionally, thermal noise is shown to decelerate the coalescence, which is confirmed by MD

(2E,6E)-2,6-Bis(2,5-difluoro­benzyl­idene)cyclo­hexa­none

In the title compound, C20H14F4O, a derivative of curcumin, the dihedral angle between the two aromatic rings is 27.19 (13)°. The C=C double bonds have an E configuration

First-principles computational investigation of nitrogen-doped carbon nanotubes as anode materials for lithium-ion and potassium-ion batteries.

Significant research efforts, mostly experimental, have been devoted to finding high-performance anode materials for lithium-ion and potassium-ion batteries; both graphitic carbon-based and carbon nanotube-based materials have been generating huge interest. Here, first-principles calculations are performed to investigate the possible effects of doping defects and the varying tube diameter of carbon nanotubes (CNTs) on their potential for battery applications. Both adsorption and migration of Li and K are studied for a range of pristine and nitrogen-doped CNTs, which are further compared with 2D graphene-based counterparts. We use detailed electronic structure analyses to reveal that different doping defects are advantageous for carbon nanotube-based and graphene-based models, as well as that curved CNT walls help facilitate the penetration of potassium through the doping defect while showing a negative effect on that of lithium

Stability of similarity solutions of viscous thread pinch-off

In this paper we compute the linear stability of similarity solutions of the breakup of viscous liquid threads, in which the viscosity and inertia of the liquid are in balance with the surface tension. The stability of the similarity solution is determined using numerical continuation to find the dominant eigenvalues. Stability of the first two solutions (those with largest minimum radius) is considered. We find that the first similarity solution, which is the one seen in experiments and simulations, is linearly stable with a complex nontrivial eigenvalue, which could explain the phenomenon of break-up producing sequences of small satellite droplets of decreasing radius near a main pinch-off point. The second solution is seen to be linearly unstable. These linear stability results compare favorably to numerical simulations for the stable similarity solution, while a profile starting near the unstable similarity solution is shown to very rapidly leave the linear regime