FengChao Wang

Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes

Graphene oxide membranes allow only very small hydrated molecules and ions to pass with an accelerated transport rate. [Also see Perspective by Mi] Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries. On the Fast Track Membranes based on graphene can simultaneously block the passage of very small molecules while allowing the rapid permeation of water. Joshi et al. (p. 752; see the Perspective by Mi) investigated the permeation of ions and neutral molecules through a graphene oxide (GO) membrane in an aqueous solution. Small ions, with hydrated radii smaller than 0.45 nanometers, permeated through the GO membrane several orders of magnitude faster than predicted, based on diffusion theory. Molecular dynamics simulations revealed that the GO membrane can attract a high concentration of small ions into the membrane, which may explain the fast ion transport.

Size effect on the coalescence-induced self-propelled droplet

An analysis based on the energy conservation is presented for the self-propelled droplet during coalescence of two droplets of the same size over a superhydrophobic rough surface. The self-propelled behavior occurs only for the coalescence of droplets with a certain range of radius. An analytical relation is established among the coalescence-induced velocity, surface energy, viscous dissipation, and droplet size if gravity is negligible. The coalescence-induced velocity increases with increasing droplet size to a maximum and then decreases with the size, which is in good accord with the experimental observation reported in the literature

Self-adaptive strain-relaxation optimization for high-energy lithium storage material through crumpling of graphene

High-energy lithium battery materials based on conversion/alloying reactions have tremendous potential applications in new generation energy storage devices. However, these applications are limited by inherent large volume variations and sluggish kinetics. Here we report a self-adaptive strain-relaxed electrode through crumpling of graphene to serve as high-stretchy protective shells on metal framework, to overcome these limitations. The graphene sheets are self-assembled and deeply crumpled into pinecone-like structure through a contraction-strain-driven crumpling method. The as-prepared electrode exhibits high specific capacity (2,165 mAh g(-1)), fast charge-discharge rate (20 A g(-1)) with no capacity fading in 1,000 cycles. This kind of crumpled graphene has self-adaptive behaviour of spontaneous unfolding-folding synchronized with cyclic expansion-contraction volumetric variation of core materials, which can release strain and maintain good electric contact simultaneously. It is expected that such findings will facilitate the applications of crumpled graphene and the self-adaptive materials.

Molecular transport through capillaries made with atomic-scale precision

Nanometre-scale pores and capillaries have long been studied because of their importance in many natural phenomena and their use in numerous applications. A more recent development is the ability to fabricate artificial capillaries with nanometre dimensions, which has enabled new research on molecular transport and led to the emergence of nanofluidics. But surface roughness in particular makes it challenging to produce capillaries with precisely controlled dimensions at this spatial scale. Here we report the fabrication of narrow and smooth capillaries through van der Waals assembly, with atomically flat sheets at the top and bottom separated by spacers made of two-dimensional crystals with a precisely controlled number of layers. We use graphene and its multilayers as archetypal two-dimensional materials to demonstrate this technology, which produces structures that can be viewed as if individual atomic planes had been removed from a bulk crystal to leave behind flat voids of a height chosen with atomic-scale precision. Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures (about 1,000 bar) and large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order in nanoconfined water. Our work opens up an avenue to making capillaries and cavities with sizes tunable to ångström precision, and with permeation properties further controlled through a wide choice of atomically flat materials available for channel walls.

Super-elastic and fatigue resistant carbon material with lamellar multi-arch microstructure.

Low-density compressible materials enable various applications but are often hindered by structure-derived fatigue failure, weak elasticity with slow recovery speed and large energy dissipation. Here we demonstrate a carbon material with microstructure-derived super-elasticity and high fatigue resistance achieved by designing a hierarchical lamellar architecture composed of thousands of microscale arches that serve as elastic units. The obtained monolithic carbon material can rebound a steel ball in spring-like fashion with fast recovery speed (∼580 mm s−1), and demonstrates complete recovery and small energy dissipation (∼0.2) in each compress-release cycle, even under 90% strain. Particularly, the material can maintain structural integrity after more than 106 cycles at 20% strain and 2.5 × 105 cycles at 50% strain. This structural material, although constructed using an intrinsically brittle carbon constituent, is simultaneously super-elastic, highly compressible and fatigue resistant to a degree even greater than that of previously reported compressible foams mainly made from more robust constituents.

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10-20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

Enhanced oil droplet detachment from solid surfaces in charged nanoparticle suspensions

The removal of oil droplets from solid surfaces is a key aspect in oil production and environmental protection. Recent progress shows that nanofluids exhibit distinct dynamic spreading behaviors compared with fluids without nanoparticles. Here, we investigated oil droplet detachment from solid surfaces immersed in charged nanoparticle suspensions via molecular dynamics simulations. Our simulated results demonstrate a significant enhancement of the oil removal efficiency using nanofluids of charged nanoparticles. When the charge on each particle exceeds a threshold value, the complete detachment of the oil droplet occurs spontaneously. Our results indicated that the surface wettability of the nanoparticles plays an essential role in oil removal processes. An increase in the interactions between nanoparticles and water molecules would obstruct the oil droplet detachment. The oil droplet detachment in the nanofluid flooding was also studied. Based on our findings, suspensions of charged hydrophobic nanoparticles can be considered to be high-performance agents in removing oil droplets from solid surfaces.

The head-on colliding process of binary liquid droplets at low velocity: high-speed photography experiments and modeling.

The experimental and theoretical studies are reported in this paper for the head-on collisions of a liquid droplet with another of the same fluid resting on a solid substrate. The droplet on the hydrophobic polydimethylsiloxane (PDMS) substrate remains in a shape of an approximately spherical segment and is isometric to an incoming droplet. The colliding process of the binary droplets was recorded with high-speed photography. Head-on collisions saw four different types of response in our experiments: complete rebound, coalescence, partial rebound with conglutination, and coalescence accompanied by conglutination. For a complete rebound, both droplets exhibited remarkable elasticity and the contact time of the two colliding droplets was found to be in the range of 10-20 ms. With both droplets approximately considered as elastic bodies, Hertz contact theory was introduced to estimate the contact time for the complete rebound case. The estimated result was found to be on the same order of magnitude as the experimental data, which indicates that the present model is reasonable.

Contact angle hysteresis at the nanoscale: a molecular dynamics simulation study

In this paper, the contact angle hysteresis (CAH) of nanodroplets on both rigid and flexible substrates with different wettabilities was studied using molecular dynamics (MD) simulations. The critical shear stress (CSS) that determines the motion of the contact line (CL) was investigated. A theoretical correlation between CAH and CSS was proposed. Both CAH and CSS reflect the energy dissipation at the CL of the droplet in response to the exerted force. MD results of CAH are qualitatively consistent with the theoretical model. Simulation results also show that, for the same liquid–solid interactions, CAH on the flexible substrate is larger than that on the rigid substrate. These findings aim to enhance our understanding of the mechanism of the CAH at the nanoscale.

Slip boundary conditions based on molecular kinetic theory: The critical shear stress and the energy dissipation at the liquid–solid interface

The boundary slip and its mechanism were investigated using theoretical analysis and molecular dynamic simulations. We proposed a new slip boundary condition based on molecular kinetic theory, which was extended by the introduction of the critical shear stress, which determines the onset of the slip, and the energy dissipation near the liquid–solid interface at high shear stress. Our results revealed that the critical shear stress increases exponentially with the liquid–solid interactions. In particular, we discussed the energy dissipation contributed from the friction between the liquid layers near the interface. The results demonstrated that the momentum transfer among the liquid layers and the bulk liquid must be considered according to the wetting conditions of the solid surface, which can interpret the contradictory published results of slip behavior at high shear stress. The analytical expression of our slip boundary condition can be compared with the simulation results since it uses parameters consistent with that used in our molecular dynamic simulations. Furthermore, we suggested a dimensionless number to qualify the transition from Poiseuille-like to plug-like flow in the carbon nanotubes. These findings can enhance our understanding of the boundary slip.