Effects of heat transfer in a growing particle layer on microstructural evolution during solidification of colloidal suspensions

Heat transfer is the foundation of freezing colloidal suspensions and a key factor for the interface movement.However,how the thermal conductivity of particles affects freezing microstructural evolution remains unknown.Here in this work,a mathematical model is built up to investigate thermal interactions among a growing particle layer,pulling speeds,and the freezing interface under a thermal gradient.Experiments are conducted to confirm the tendency predictions of the model.With the increase of pulling speeds,the drifting distance of the freezing interface increases and the time to finish drifting decreases.When the thermal conductivity of particles(k_(p))is smaller than that of the surrounding(kw),the freezing interface tends to go forward to the warm side.Contrarily,the freezing interface tends to go back to the cold side when the thermal conductivity of particles is larger than that of the surrounding(α=k_(p)/k_(w)>1).It originates from the shape of the local freezing interface:convex(α<1)or concave(α>1).These morphological changes in the local interface modify the premelting drag force F_(f).Whenα<1,F_(f)decreases and the freezing morphology tends to be the frozen fringe.Whenα>1,F_(f)increases and the freezing morphologies tend to be ice spears.These understandings of how the thermal conductivity of particles affect microstructural evolution may optimize the production of freeze-casting materials and their structural-functional properties.

Can secondary nucleation exist in ice banding of freezing colloidal suspensions?

The formation mechanism of ice banding in the system of freezing colloidal suspensions, which is of significance in frost heaving, ice-templating porous materials and biological materials, is still a mystery. Recently, the theory of secondary nucleation and growth of ice has been proposed to explain the emergence of a new ice lens. However, this theory has not been quantitatively examined. Here, we quantitatively measure the initial interfacial undercooling of a new ice lens and the nucleation undercoolings of suspensions. We find that the interfacial nndercooling cannot satisfy the nucleation undercooling of ice and hence disprove the secondary nucleation mechanism for ice banding.

Transport coefficients and mechanical response in hard-disk colloidal suspensions

We investigate the transport properties and mechanical response of glassy hard disks using nonlinear Langevin equation theory.We derive expressions for the elastic shear modulus and viscosity in two dimensions on the basis of thermalactivated barrier-hopping dynamics and mechanically accelerated motion.Dense hard disks exhibit phenomena such as softening elasticity,shear-thinning of viscosity,and yielding upon deformation,which are qualitatively similar to dense hard-sphere colloidal suspensions in three dimensions.These phenomena can be ascribed to stress-induced ＂landscape tilting＂.Quantitative comparisons of these phenomena between hard disks and hard spheres are presented.Interestingly,we find that the density dependence of yield stress in hard disks is much more significant than in hard spheres.Our work provides a foundation for further generalizing the nonlinear Langevin equation theory to address slow dynamics and rheological behavior in binary or polydisperse mixtures of hard or soft disks.

Preparation of stable colloidal suspensions of superdisintegrants via wet stirred media milling

Superdisintegrants are cross-linked polymers that can be used as dispersants for fast release of drug nanoparticles from nanocomposite microparticles during in vitro and in vivo dissolution. Currently avail- able superdisintegrant particles have average sizes of approximately 5-130 μm, which are too big for drug nanocomposite applications. Hence, production of stable superdisintegrant suspensions with less than 5 μm particles is desirable. Here, we explore the preparation of colloidal suspensions of anionic and nonionic superdisintegrants using a wet stirred media mill and assess their physical stability. Sodium starch glycolate （SSG） and crospovidone （CP） were selected as representative anionic and nonionic superdisintegrants, and hydroxypropyl cellulose （HPC） and sodium dodecyl sulfate （SDS） were used as a steric stabilizer and a wetting agent/stabilizer, respectively. Particle sizing, scanning electron microscopy, and zeta potential measurements were used to characterize the suspensions. Colloidal superdisintegrant suspensions were prepared reproducibly. The extensive particle breakage was attributed to the swelling-induced softening in water. SSG suspensions were stable even in the absence of stabilizers, whereas CP suspensions required HPC-SDS for minimizing particle aggregation. These findings were explained by the higher absolute （negative） zeta potential of the suspensions of the anionic superdisintegrant （SSG） as compared with those of the nonionic superdisintegrant （CP）.

Determination of Particle Sizes and Crystalline Phases on Colloidal Silicon Nanoparticle Suspensions

Particle size and crystallinity of silicon nanoparticles were determined by analyzing the optical extinction spectra of colloidal suspensions. Experimental results from these colloids were anaiyzed using Mie theory in connection with effective medium theory, in order to determine particle sizes and their internal structure with the simple technique of optical transmission spectroscopy. By modeling an effective refractive index for the particles, the crystalline volume fraction can be extracted from extinction spectra in addition to information about the size. The crystalline volume fraction determined in this way were used to calibrate the ratio of the Raman cross sections for nanocrystalline and amorphous silicon, which was found to be σc./σa = 0.66

Suspension mechanism and application of sand-suspended slurry for coalmine fire prevention

North and west China has abundant coal resources, however, such resources make these regions prone to serious mine fire disasters. Although the copious sand and fly ash resources found in these areas can be used as fire-fighting materials, conventional grouting is expensive because of water shortage and loess particles. A new compound material(i.e., a sand-suspended colloid), which comprises a mineral inorganic gel and an organic polymer, is developed in the current study to improve the quality of sand injection and reduce water wastage when grouting. The new material can steadily suspend the sand, through the addition of a small amount of colloid yielding steady sand-suspended slurry. The process of producing the slurry is convenient and quick, overcoming the shortage of sand-suspending thickeners which need heat and are difficult to produce. The space work model based on the theory of the double-electric layer is established to study the suspended mechanism of the solid particles in the sand-suspended colloid.The dispersion effect of the sand-suspended colloid is demonstrated by the incorporation of the electrostatic effect by the double-electric layer and the steric hindrance effect on the sand particles, ensuring the stability of the colloid system and the steady suspension of sand particles in the sand-suspended colloid.Mechanical analysis indicates that the sand is suspended steadily under the condition that the rock sand particles stress on the lower part of the fluid is less than the yield stress of the colloid. Finally, the fireprevention technology of sand suspension was applied and tested in the Daliuta Coal Mine, achieving successful results.

Prediction of aggregation behavior of submicron-sized particles of praseodymium-doped zirconium silicate in aqueous suspension by population balance model

The aggregation behavior of submicron-sized particles of praseodymium-doped zirconium silicate, a ceramic pigment, in aqueous suspension was predicted by a modified population balance model, In the model, the collision frequencies were selected to describe evolution of the particle size distribution of the suspension. The collision efficiency was estimated as a function of interaction potential between particles based on Derjaguin-Landau-Verwey-Overbeek theory. The population balance model was modified to predict the stable state of the aggregation by introducing the volume mean size of aggregate to stability ratio. In addition, aggregation of the particles in aqueous suspension in the presence of sodium dodecyl benzene sulfonate or potassium chloride was experimentally investigated. The predicted data （i.e., the final aggregate size, aggregation rate, and particle size distribution） were similar to the experimentalresults.

Precise synthesis of discrete and dispersible carbon- protected magnetic nanoparticles for efficient magnetic resonance imaging and photothermal therapy

Carbon-protected magnetic nanoparticles exhibit long-term stability in acid or alkaline medium, good biocompatibility, and high saturation magnetization. As a result, they hold great promise for magnetic resonance imaging, photothermal therapy, etc. However, since pyrolysis, which is often required to convert the carbon precursors to carbon, typically leads to coalescence of the nanoparticles, the obtained carbon-protected magnetic nanoparticles are usually sintered as a non-dispersible aggregation. We have successfully synthesized discrete, dispersible, and uniform carbon-protected magnetic nanoparticles via a precise surface/interface nano-engineering approach. Remarkably, the nanoparticles possess excellent water-dispersibility, biocompatibility, a high T2 relaxivity coefficient （384 mM^-1·s^-1）, and a high photothermal heating effect. Furthermore, they can be used as multifunctional core components suited for future extended investigation in early diagnosis, detection and therapy, catalysis, separation, and magnetism.

Zeta potential measurement on lithium lanthanum titanate nanoceramics

The zeta potential, isoelectric point, and agglomeration of Lio.sLao.sTiO3 （LLTO） nanoparticles dispersed in aqueous media at different ionic strengths have been studied. The zeta potential was determined from electrophoretic mobility measurements, according to Smoluchowski＇s equation, for Li0.5La0.5TiO3 suspen- sions in NaCl and KCI electrolytes with ionic strengths of 1, 10, and 100 mmol/dm3. The isoelectric point （IEP）, zeta potential （ζ）, and the agglomeration were shown to strongly depend on the ionic strength of the Li0.5La0.5TiO3 aqueous colloidal suspension in both NaCI and KCI electrolytes, which allows the deter- mination of the effects of environmental conditions for Lio.sLao.sTiO3 manipulation in aqueous colloidal systems. The suspensions of Li0.5La0.5TiO3 nanoparticles reach the IEP in the pH range 0f3-5. The （ of Li0.5La0.5TiO3 nanoparticles varied from positive to negative values with a pH increase, which allows for the control of the surface charge depending on the purpose. The pH range of 7-g and an ionic strength ≤1 mmol/dm3 are recommended as the most suitable conditions for both the LLTO colloidal shaping techniques application and the LLTO-based nanocomposite formation.

High-efficiency laser-irradiation spheroidizing of NiCo_2O_4 nanomaterials

We realized the desired spheroidizing of NiCo_2O_4 nanomaterials by laser irradiating NiCo_2O_4 suspensions with different concentrations. The results reveal that the as-prepared samples are desired spheres with the maximal average size of 568 nm and the superior dispersity, which were obtained at the energy density of 0.30 J·pulse^(-1)·cm^(-2) and NiCo_2O_4 suspension concentration of 0.2 mg·mL^(-1). However, the phase segregation, which was induced by large amounts of solid redox of Co^(3+)/Co^(2+) and Ni^(3+)/Ni^(2+), also appears in the laser-irradiation process.