Fluid transport that is driven by gradients of pressure, gravity, or electro-magnetic potential is well-known and studied in many fields. A subtler type of transport, called diffusiophoresis, occurs in a gradient of chemical concentration, either electrolyte or non-electrolyte. Diffusiophoresis works by driving a slip velocity at the fluid-solid interface. Although the mechanism is well-known, the diffusiophoresis mechanism is often considered to be an esoteric laboratory phenomenon. However, in this article we show that concentration gradients can develop in a surprisingly wide variety of physical phenomena - imposed gradients, asymmetric reactions, dissolution, crystallization, evaporation, mixing, sedimentation, and others - so that diffusiophoresis is in fact a very common transport mechanism, in both natural and artificial systems. We anticipate that in georeservoir extractions, physiological systems, drying operations, laboratory and industrial separations, crystallization operations, membrane processes, and many other situations, diffusiophoresis is already occurring - often without being recognized - and that opportunities exist for designing this transport to great advantage.

Dead-end micro- and nanoscale channels are ubiquitous in nature and are found in geological and biological systems subject to frequent disruptions. Achieving fluid flows in them is not possible through conventional pressure-driven mechanisms. Here we show that chemically driven convective flows leading to transport in and out of dead-end pores can occur by the phenomenon of "transient diffusioosmosis". The advective velocity depends on the presence of an in situ-generated transient ion gradient and the intrinsic charge on the pore wall. The flows can reach speeds of 50 μm/s and cause extraction of otherwise-trapped materials. Our results illustrate that chemical energy, in the form of a transient salt gradient, can be transduced into mechanical motion with the pore wall acting as the pump. As discussed, the phenomena may underlie observed transport in many geological and biological systems involving tight or dead-end micro- and nanochannels.

Calcium carbonate particles, ubiquitous in nature and found extensively in geological formations, behave as micropumps in an unsaturated aqueous solution. The mechanism causing this pumping is diffusioosmosis, which drives flows along charged surfaces. Our calcium carbonate microparticles, roughly ∼10 μm in size, self-generate ionic gradients as they dissolve in water to produce Ca(2+), HCO(3)(-), and OH(-) ions that migrate into the bulk. Because of the different diffusion coefficients of these ions, spontaneous electric fields of roughly 1-10 V/cm arise in order to maintain electroneutrality in the solution. This electric field drives the diffusiophoresis of charged tracers (both positive and negative) as well as diffusioosmotic flows along charged substrates. Here we show experimentally how the directionality and speed of the tracers can be engineered by manipulating the tracer zeta potential, the salt gradients, and the substrate zeta potential. Furthermore, because the salt gradients are self-generated, here by the dissolution of solid calcium carbonate microparticles another manipulated variable is the placement of these particles. Importantly, we find that the zeta potentials on surfaces vary with both time and location because of the adsorption or desorption of Ca(2+) ions; this change affects the flows significantly.

Ultraviolet (UV) and orange emissions have been observed from vapor-liquid-solid grown SnO2 nanowires. From the luminescence, the donor and acceptor binding energies have been estimated. The dependence of the orange luminescence on the diameters of tin oxide nanowires has been observed and the wavelength of the UV luminescence is found to depend on the laser power. Both the shift in the UV and the intensity of the orange luminescence is found to be dependent on the surface states of the tin oxide nanowires.

Luminescence data obtained in the visible region from the SnO 2 nanowires are used to determine the defect levels within the bandgap responsible for the strong orange emission, and a shift in the orange luminescence is seen at low temperatures. Temperature-dependent photoluminescence (PL) in the UV region shows the merging of the various emission lines at low temperature into a single broad UV peak with increasing temperatures. Investigations of PL and transmission electron microscope-energy dispersive x-ray spectroscopy (TEM-XEDS) from wires of different diameters show that the luminescence in the gap originates from surface states and thinner wires have more oxygen vacancies compared to thicker ones. Nanowires post rapid thermal annealing in two different ambients viz. oxygen and nitrogen are compared, using Raman, photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS) as characterization tools. Our data demonstrate that annealing in oxygen improves the crystalline quality of the nanowires due to the decrease in the oxygen vacancies.