Electric field mediated separation of water-ethanol mixtures in carbon-nanotubes integrated in nanoporous graphene membranes.

We investigate the influence of an applied electric field on the separation of a water-ethanol solution inside a carbon nanotube (CNT) using a series of molecular dynamics simulations. The electric field is applied at an angle θ with respect to the axis of the CNT. The study uncovers that with the application of a 'small-angle' electric field (e.g. smaller θ), the water molecules exhibit preferential occupancy inside the CNT, whereas the application of the same electric field at a 'wide-angle' mode (e.g. higher θ) fills the CNT with ethanol molecules in place of water. Remarkably, the direction of the electric field plays a pivotal role because the field exerts a contrasting influence on the behaviours of the water and ethanol molecules. The water dipoles are favourably aligned at small values of θ creating an ordered water structure inside the CNT. Increasing θ disrupts the water dipole orientation and leads to the preferential occupancy of the CNT by ethanol molecules. An in-depth analysis on the simulated systems unveil that, at lower values of θ, multiple layers of water molecules are physically adsorbed near the CNT walls, which is found to diminish as θ is increased. In comparison, at higher magnitudes of θ, the ethanol molecules are preferentially adsorbed inside the CNT. The average interaction energy per ethanol (water) molecule is found to increase (reduce) when θ is monotonically increased, which can be ascribed to the increase (decrease) in the intermolecular hydrogen bonding capacity of the ethanol (water) molecules at larger values of θ. Consequently, inside the CNT, the average occupancy of water molecules decreases and ethanol molecules increases, as θ is monotonically increased, leading to the separation of the ethanol-water mixture. The proposed methodology can convert an equimolar mixture (1 : 1) of ethanol-water into a concentrated one (14 : 1) when the electric field is applied orthogonal to the axis of the CNT. The separation efficiency is found to improve with an increase in the intensity of the externally applied electric field.

Electric field mediated spraying of miniaturized droplets inside microchannel.

We report a facile and noninvasive way to disintegrate a microdroplet into a string of further miniaturized ones under the influence of an external electrohydrodynamic field inside a microchannel. The deformation and breakup of the droplet was engendered by the Maxwell's stress originating from the accumulation of induced and free charges at the oil-water interface. While at smaller field intensities, for example less than 1 MV/m, the droplet deformed into a plug, at relatively higher field intensities, e.g. ∼1.16 MV/m, a pair of droplets having opposite surface charge was formed. The charged droplets showed an interesting periodic bridging and breakup during their translation motion across the channel. For even higher field intensities, for example more than 1.2 MV/m, the entire droplet underwent dielectrophoresis toward one of the electrodes before experiencing a strong attractive force from the other electrode to deform into a shape of a Taylor cone. With progress in time, mimicking the electrospraying phenomenon, the cone tip periodically ejected a string of miniaturized water droplets to form a microemulsion inside the channel. The frequency and size of the droplet ejection could be tuned by varying the applied field intensity. A water droplet of ∼214 µm diameter could continuously eject droplets of size ∼10 µm or even smaller to form a microemulsion inside the channel.

Dynamics of deformation and pinch-off of a migrating compound droplet in a tube.

A computational fluid dynamic investigation has been carried out to study the dynamics of a moving compound droplet inside a tube. The motions associated with such a droplet is uncovered by solving the axisymmetric Navier-Stokes equations in which the spatiotemporal evolution of a pair of twin-deformable interfaces has been tracked employing the volume-of-fluid approach. The deformations at the interfaces and their subsequent dynamics are found to be stimulated by the subtle interplay between the capillary and viscous forces. The simulations uncover that when a compound drop composed of concentric inner and outer interfaces migrates inside a tube, initially in the unsteady domain of evolution, the inner drop shifts away from the concentric position to reach a morphology of constant eccentricity at the steady state. The coupled motions of the droplets in the unsteady regime causes a continuous deformation of the inner and outer interfaces to obtain a configuration with a (an) prolate (oblate) shaped outer (inner) interface. The magnitudes of capillary number and viscosity ratio are found to have significant influence on the temporal evolution of the interfacial deformations as well as the eccentricity of the droplets. Further, the simulations uncover that, following the asymmetric deformation of the interfaces, the migrating compound droplet can undergo an uncommon breakup stimulated by a rather irregular pinch-off of the outer shell. The breakup is found to initiate with the thinning of the outer shell followed by the pinch-off. Interestingly, the kinetics of the thinning of outer shell is found to follow two distinct power-law regimes-a swiftly thinning stage at the onset followed by a rate limiting stage before pinch-off, which eventually leads to the uncommon breakup of the migrating compound droplets.

Formation of liquid drops at an orifice and dynamics of pinch-off in liquid jets.

This paper presents a numerical investigation of the dynamics of pinch-off in liquid drops and jets during injection of a liquid through an orifice into another fluid. The current study is carried out by solving axisymmetric Navier-Stokes equations and the interface is captured using a coupled level-set and volume-of-fluid approach. The delicate interplay of inertia and viscous effects plays a crucial role in deciding the dynamics of the formation as well as breakup of liquid drops and jets. In the dripping regime, the growth and breakup rate of a drop are studied and quantified by corroborating with theoretical predictions. During the growth stage of the drops, a self-similar behavior of the drop profile is identified over a relatively short duration of time. The viscosity of the drop liquid shows substantial influence on the thinning behavior of a liquid neck and a transition is observed from an inertia dominated regime to an inertia-viscous regime beyond a critical minimum value of the neck radius. The phenomenon of interface overturning is fundamentally related to the magnitude of drop viscosity. The variation of overturning angle as a function of drop viscosity is computed and a critical value of Ohnesorge number is obtained beyond which overturning ceases. Increasing the inertia of drop liquid transforms the system from a periodically dripping regime to a quasiperiodic regime and finally it culminates into an elongated liquid jet. Another interesting transition from dripping to jetting regime is demonstrated by varying the viscosity of the ambient medium. The breakup of jets in Rayleigh mode is explored and the breakup length obtained from our computations shows excellent agreement with the theoretical predictions owing to Rayleigh's analysis. The ambient medium is entrained as the jet moves downstream with the creation of a vortical structure just outside the jet signifying increased participation of the ambient medium in the dynamics of jet breakup at higher inflow rates.