Experimental investigation of heat conduction mechanisms in nanofluids. Clue on clustering

Heat conduction mechanisms in nanofluids, fluids seeded with nanoparticles, have been extensively scrutinized in the past decades to explain some experimental observations of their enhanced thermal conductivity beyond the effective medium theory. Although many mechanisms such as Brownian motion, clustering, ballistic transport, and internanoparticle potential are speculated, experimental proof of any of the mechanisms has been difficult. Here, we investigate the mechanisms experimentally by thermal conductivity measurements and structural analysis for the same materials in both liquid and solid states. These studies strongly suggest that clustering holds the key to the thermal conductivity enhancement of nanofluids.

Influence of organic acids on the transport of heavy metals in soil

Vegetation historically has been an important part of reclamation of sites contaminated with metals, whether the objective was to stabilize the metals or remove them through phytoremediation. Understanding the impact of organic acids typically found in the rhizosphere would contribute to our knowledge of the impact of plants in contaminated environments. Heavy metal transport in soils in the presence of simple organic acids was assessed in two laboratory studies. In the first study, thin layer chromatography (TLC) was used to investigate Zn, Cd, and Pb movement in a sandy loam soil as affected by soluble organic acids in the rhizosphere. Many of these organic acids enhanced heavy metal movement. For organic acid concentrations of 10mM, citric acid had the highest R(f) values (frontal distance moved by metal divided by frontal distance moved by the solution) for Zn, followed by malic, tartaric, fumaric, and glutaric acids. Citric acid also has the highest R(f) value for Cd movement followed by fumaric acid. Citric acid and tartaric acid enhanced Pb transport to the greatest degree. For most organic acids studied, R(f) values followed the trend Zn>Cd>Pb. Citric acid (10mM) increased R(f) values of Zn and Cd by approximately three times relative to water. In the second study, small soil columns were used to test the impact of simple organic acids on Zn, Cd, and Pb leaching in soils. Citric acid greatly enhanced Zn and Cd movement in soils but had little influence on Pb movement. The Zn and Cd in the effluents from columns treated with 10mM citric acid attained influent metal concentrations by the end of the experiment, but effluent metal concentrations were much less than influent concentrations for citrate <10mM. Exchangeable Zn in the soil columns was about 40% of total Zn, and approximately 80% total Cd was in exchangeable form. Nearly all of the Pb retained by the soil columns was exchangeable.

Fast photorefractive response in strongly reduced near-stoichiometric LiNbO(3) crystals

The photorefractive response time of LiNbO(3) crystal is of the order of minutes, and such a long response time limits the crystal's practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm (2) , and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO(3) crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.