a Size fractionation, amplified by the surface charge density of graphene oxide (GO) sheets, broadens the pH dependent isotropic (I) to nematic (N) phase transition in aqueous dispersions of graphene oxide (GO). In this biphasic region, a highly organized droplet nematic phase of uniform size (20 ± 2.8 μm diameter) with an isotropic interior is observed.Suspensions of 2D sheet-like materials exhibit transition from a disordered isotropic (I) phase to ordered nematic (N) phase. As noted by Onsager, 1 despite their loss of orientational entropy, the N phase, is stabilized by a gain in excluded volume (or configurational) entropy. Graphene -a single atom thick layer of carbon atoms is receiving immense attention owing to the combination of properties such as large intrinsic mobility of electrons, massive surface area, immense mechanical strength and large thermal conductivity. Although significant progress has been made in the solid-state synthesis of graphene, liquid-phase graphene possess opportunities for large-scale synthesis and novel mesophases that may lead to ordered macroscopic structures such as thin films and fibers with enhanced electrical, mechanical and optical properties.2, 3 Graphene solvated in chlorosulfonic acid 2 and stabilized by surfactant 4 exhibit liquid crystalline behavior, as well as their composites with discotic molecules.5 Chemically oxidized graphene sheets or graphene-oxide (GO) suspensions in water, the most accessible precursor to graphene-based materials, not surprisingly demonstrate I-N phase transitions, too.6-10 These studies have shown that the nematic phases of graphene exhibit 'brush-like' texture in massively organized continuous domains that may reach length-scales of millimeters. These length-scales are relevant to achieving macroscopcially ordered materials and fluid-phase alignment. However, graphene sheets are flexible and although this flexibility is likely to yield isolated droplet liquid crystalline phases, yet such confined phases have not been reported so far. The ability to form highly ordered N phases from GO arises because GO can be stabilized in aqueous solvent by electrostatic repulsive forces between the particles as demonstrated by large (-20 to -40 mV) zeta potential measurements.11-13 The charged functional groups are primarily carboxylic groups on the edges and hydroxyls on the basal planes, which can be protonated/deprotonated by change in pH, thus surface charging of these particles depend on the pH. pKa of GO is ~ 4 for the carboxylic groups and ~9 for the phenolic groups, 14 however GO suspensions can be stable at lower pH through hydrogen bonding between the oxidized groups and water molecules. 15 In addition, the basal plane contains hydrophobic domains that make GO sheets amphiphilic, and the ionisable groups render this amphiphilicity a dependence on pH and size of the GO sheets with smaller GO sheets being more hydrophilic than larger GO sheets. 13,[16][17][18] It can thus be expected that the I-N phase transitions are dependent upon factors suc...

7861wileyonlinelibrary.com and electrical double layer overlapping and its effect on charge permselectivity and electroosmosis must be taken into account. They are indeed essential for applications, for example, detection of individual DNA molecules, [11] DNA stretching analysis, [12] energy conversion/ storage, [13][14][15] water purification, [16,17] and the investigation of liquid properties, [18] all very difficult to achieve in a microfluidic platform. However, the large surface and viscous forces that resist fluid motion at very small scales are associated with a low Reynolds number in most micro-or nanofluidic systems. Such low Reynolds number hydrodynamics pose significant challenges, not the least of which are the low fluid transport speeds that undermine micropumping [19] and the difficulty in generating turbulent vortices and irregular fluid flow required for micromixing. [20,21] In addition, fluid in microchannels is often manipulated with hydrostatic pressure, achieving well-controlled fluid flow remains a challenge. [22][23][24] The scenario worsens when it comes to pressure-driven flow in nanochannels with the requirement of much higher pressure [25,26] to produce fluid flow due to the power-law relation between the pressure and the cross-sectional size of the channel. [27] Chip-based fluidic actuators using surface acoustic waves (SAW) have become popular among microfluidic practitioners who continue to explore new applications in acoustofluidic integration. [28,29] Planar devices employing lithium niobate in conjunction with deposited and patterned interdigital electrodes (IDTs) were first reported in the 1960s by White and Voltmer, [30] delivering Rayleigh SAW as a nanometer-order amplitude electromechanical wave propagating from the IDT. [28] One of the most attractive aspects of using SAW for microfluidic actuation and manipulation is their very efficient fluidstructural coupling: most of the energy in the substrate is adjacent the solid-fluid interface, within four to five wavelengths of SAW from the surface. When SAWs come into contact with a fluid in its wave path, energy is leaked from the SAW into the fluid to form sound propagating at the Rayleigh angle from the solid-fluid interface owing to the mismatch of sound velocities between the fluid and the SAW in the substrate. [28] Most importantly, through the combination of acoustic streaming and direct acoustic forces on objects in the fluid, the MHz-order Controlled nanoscale manipulation of fluids and colloids is made exceptionally difficult by the dominance of surface and viscous forces. Acoustic waves have recently been found to overcome similar problems in microfluidics, but their ability to do so at the nanoscale remains curiously unexplored. Here, it is shown that 20 MHz surface acoustic waves (SAW) can manipulate fluids, fluid droplets, and particles, and drive irregular and chaotic fluid flow within fully transparent, high-aspect ratio 50-250 nm tall nanoslits fabricated via a new direct, room temperature bonding method for lithiu...

We report capillary-force-assisted self-assembly (CAS) as a method for preparation of thin films of chemically reduced graphene oxide (rGO) with unidirectional organization of rGO sheets. The films were initiated at the contact line of the air–liquid–solid interface and form directly on solid substrates dipped in an isotropic colloidal suspension of rGO. Assisted by capillary forces at the contact line, the suspension undergoes an isotropic-to-anisotropic phase transition and becomes aligned with the film growth direction as the contact line moves across the substrate surface. We determined the degree of order in rGO films and assemblies by birefringence and diattenuation imaging. The slow axis of the rGO platelets within the CAS films displayed a narrow angular distribution (±3°) within a film area of 1 mm2, resulting in the highest possible order parameter (S) of ∼1 with 8-fold enhancement of electrical conductivity compared to films formed by traditional techniques such as filtration. Our straightforward film fabrication technique is scalable to produce large areas of films, and by controlling the rates of convective to diffusive mass transport, films with varying degree of order can be produced.