Denitsa Milanova

Role of ions in pool boiling heat transfer of pure and silica nanofluids

Heat transfer in silica nanofluids at different acidity and base is measured for various ionic concentrations in a pool boiling experiment. Nanosilica suspension increases the critical heat flux 3 times compared to conventional fluids. The 10-nm particles possess a thicker double diffuse layer compared to 20-nm particles. The catalytic properties of nanofluids decrease in the presence of salts, allowing the particles to cluster and minimize the potential increase in heat transfer. Nanofluids in a strong electrolyte, i.e., in high ionic concentration, allow a higher critical heat flux than in buffer solutions because of the difference in surface area. The formation and surface structure of the deposition affect the thermal properties of the liquid.

Heat Transfer Behavior of Silica Nanoparticles in Pool Boiling Experiment

The heat transfer characteristics of silica (SiO 2 ) nanofluids at 0.5 vol % concentration and particle sizes of 10 nm and 20 nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The influence of acidity on heat transfer has been studied. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When there is no particle deposition on the wire, the nanofluid increases critical heat flux (CHF) by about 50% within the uncertainty limits regardless of pH of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. This deposition occurs for the charged 10 nm silica particle. The chemical composition, oxidation, and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the burnout heat flux.

Effect of surface tension on nanotube nanofluids

This letter presents heat transfer results that single-walled carbon nanotube (CNT) suspensions in a boiling environment can extend the saturated boiling regime and postpone catastrophic failure of the material even further than previously reported if the surface tension of the nanofluid is carefully controlled. The maximum enhancement in the critical heat flux is nearly four times for a surfactant to CNT concentration ratio of 1:5. The experimental results show that the material burnout is a strong function of the relaxation of the nanofluid surface tension with the base fluid.

Electrophoretic mobility measurements of fluorescent dyes using on-chip capillary electrophoresis

We present an experimental study of the effect of pH, ionic strength, and concentrations of the electroosmotic flow (EOF)‐suppressing polymer polyvinylpyrrolidone (PVP) on the electrophoretic mobilities of commonly used fluorescent dyes (fluorescein, Rhodamine 6G, and Alexa Fluor 488). We performed on‐chip capillary zone electrophoresis experiments to directly quantify the effective electrophoretic mobility. We use Rhodamine B as a fluorescent neutral marker (to quantify EOF) and CCD detection. We also report relevant acid dissociation constants and analyte diffusivities based on our absolute estimate (as per Nernst–Einstein diffusion). We perform well‐controlled experiments in a pH range of 3–11 and ionic strengths ranging from 30 to 90 mM. We account for the influence of ionic strength on the electrophoretic transport of sample analytes through the Onsager and Fuoss theory extended for finite radii ions to obtain the absolute mobility of the fluorophores. Lastly, we briefly explore the effect of PVP on adsorption–desorption dynamics of all three analytes, with particular attention to cationic R6G.

An Experimental Study of Surface Tension-Dependent Pool Boiling Characteristics of Carbon Nanotubes-Nanofluids

This paper reports an experimental investigation of the pool boiling heat transfer characteristics of single-walled carbon nanotubes (SWCNTs)-nanofluids. Two main characteristics were studied to identify their influence on boiling heat transfer: one is the surface tension through the addition of surfactant and the other is the chemical treatment of nanotubes sidewalls (i.e. oxidized and untreated sidewalls). A Transmission Electron Microscope was used to study the morphology of the functionalized nanotubes and their deposition on heater wire. The maximum enhancement of both the critical and burnout heat fluxes of this nanofluid over those of the pure deionized water are found to be 492% and 265%, respectively at a surfactant to carbon nanotubes concentration ratio of 1:5. This indicates that high enhancement of heat flux is possible and would depend on the concentration of the surfactants. Present results also demonstrate that CNT-nanofluids in a pool boiling environment can extend the saturated boiling regime and the burnout of the heated surface. The burnout heat flux is found to be a strong function of the relaxation of nanofluid surface tension with the base fluid. Based on the best fit of experimental data, an empirical correlation between the burnout heat flux of nanofluid and its relaxation of surface tension is introduced.© 2009 ASME

Effect of PVP on the electroosmotic mobility of wet-etched glass microchannels

We present an experimental study on the effect of polymer PVP on EOF mobility of microchannels wet etched into optical white soda lime glass, also known as Crown glass. We performed experiments to evaluate the effect of PVP concentration and pH on EOF mobility. We used on‐chip capillary zone electrophoresis and a neutral fluorescent dye as a passive marker to quantify the electroosmotic flow. We performed experiments under controlled conditions by varying pH from 5.2 and 10.3 and concentration of PVP from 0 to 2.0% w/w at constant ionic strength (30 mM). Our experiments show that PVP at concentrations of 1.0% or above very effectively suppress EOF at low pH (6.6). At high pH of 10.3, PVP has a much weaker suppressing effect on EOF and increasing its concentration above about 0.5% showed negligible effect on EOF mobility. Finally, we briefly discuss the effects of pH on using PVP as an adsorbed coating. Our experiments provide useful guidelines on choosing correct pH and concentration of PVP for effective EOF suppression in glass channels.

Heat Transfer Behavior of Oxide Nanoparticles in Pool Boiling Experiment

The heat transfer characteristics of silica (SiO2 ), ceria (CeO2 ), and alumina (Al2 O3 ) nanofluids at 0.5% concentration and particle size of 10nm and 20 nm in pool boiling have been analyzed. The influence of acidity on heat transfer has been studied. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles, their mutual interactions towards the wire. When there is no particle deposition on the wire, the nanofluid with any oxide suspension increases CHF by about 50% within uncertainty limits regardless of the type of the oxide particle and its size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. Amorphous oxides (SiO2 ) are generally more disordered and less closely packed compared to the crystalline oxides such as CeO2 and Al2 O3 . The arrangement of the atoms within the unit cell and the layer of water molecules at the surface possibly influence the natural convection as well as the CHF. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly 3-fold increase in CHF. This deposition occurs for the charged 10 nm silica particle, and was not seen for other oxide particles. The chemical composition, oxidation and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the Critical Heat Flux.Copyright

Dispersion and surface characteristics of nanosilica suspensions.

Nanofluids consisting of nanometer‐sized particles dispersed in base liquids are known to be effective in extending the saturated boiling regime and critical heat flux in pool boiling. The heat transfer characteristics of nanosilica suspensions with particle sizes of 10 and 20 nm in pool boiling with a suspended heating Nichrome wire have been analyzed. The pH value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When silica is suspended in water with no additives, the surface potential of the nanoparticles determines their movement toward the electrodes. Particles continuously deposit on the wire and extend the burnout heat flux, influenced by the chemical composition of the nanofluids. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. Particle size, zeta potential, and the burnout heat flux values under different volume concentrations are provided. The burnout heat flux of the wire does not increase monotonically with concentration, but depends on the agglomeration characteristics, particle shape, and the hydroxylated surface of the nanoparticles.

Functionalized Single Walled and Double Walled Carbon Nanotubes for Thermal Enhancement

Our investigation in this study focuses on the chemical and thermal characteristics of Single Walled Nanotubes (SWNTs) in comparison with Double Walled Nanotubes (DWNTs) for their potential to extend the heat flux in the boiling regime. SWNTs and DWNTs were successfully dispersed in an aqueous medium by reducing the surface tension, i.e. adsorption of anionic surfactant (NaDDBS). Two mechanisms of stabilization have been adopted: electrosteric and steric stabilization, and the suspensions were seen to be stable for longer periods. DWNTs have better heat transfer performance in the pool boiling experiment due to their unique structure. Functionalization and surfactant coating of SWNTs may reduce the heat flow because of modification of the structure. The burnout heat flux of the immersed Nichrome wire in water under pool boiling conditions was increased by up to 170% over that for the pure base liquid for DWNT nanofluid. In contrast, SWNT nanofluid enhanced burnout heat flux by 30%. The nature of the deposition on the heating element affects the surface wetting and therefore the pool boiling curve. DWNT coating on the wire is much thinner than SWNT coating; however, it is more porous and enhances CHF.Copyright

An enhanced CRISPR repressor for targeted mammalian gene regulation

The RNA-guided endonuclease Cas9 can be converted into a programmable transcriptional repressor, but inefficiencies in target-gene silencing have limited its utility. Here we describe an improved Cas9 repressor based on the C-terminal fusion of a rationally designed bipartite repressor domain, KRAB–MeCP2, to nuclease-dead Cas9. We demonstrate the system’s superiority in silencing coding and noncoding genes, simultaneously repressing a series of target genes, improving the results of single and dual guide RNA library screens, and enabling new architectures of synthetic genetic circuits.The fusion of dead Cas9 with KRAB and the transcriptional repressor domain of the chromatin modifier MeCP2 leads to an efficient transcriptional silencer that can be applied to genome-scale screens and genetic circuits.