Imaging droplet freezing using MRI

Magnetic resonance imaging (MRI) is used to provide spatially resolved structural and chemical composition characterisation of droplets undergoing freezing. To this end, MRI is applied to a diameter sucrose solution droplet, suspended in cold air. During the consequential solidification of the droplet, the spatial location of nucleation and the crystal growth of the droplet are followed using non-invasive two dimensional (2D) images; these are produced using the fast MRI technique, RARE. This is able to both quantify crystal growth rates, as well as the unfrozen liquid mass fraction for the optically opaque freezing droplets. Such information is of major interest in the verification of models describing the freezing of such droplets. The spatial re-distribution of the sucrose solute as a consequence of freezing is monitored using MR 1D chemical shift profiling. The formation of a concentrated sucrose layer at the droplet surface was detected.     

Dynamic rheological behavior and morphology near phase-separated region for a LCST-type of binary polymer blends

The dynamic rheological properties and morphology in the vicinity of phase-separated region for poly(methyl methacrylate) (PMMA)/poly(styrene-co-acrylonitrile) (SAN) blends with lower critical solution temperature (LCST) behavior were investigated. When temperature was above the phase separation temperature, i.e. cloud point (T_c) for some PMMA/SAN blends, the slope of plotting versus decreased at low frequencies (terminal region), indicating the appearance of phase-separation and existence of heterogeneous structure. We employed a model dealing with complex modulus of the two phases mixture proposed by Kopnistos et al. for describing the dynamic rheological behaviors of PMMA/SAN blends, according to the assumption that the interfacial tension between the matrix and the dispersed phase was independent of local shear and variation of interfacial area, and that the dispersed spherical droplets were nearly monodispersed. It is found that the predicted results were in qualitative agreement with the experimental data of this study. The ratio of interfacial tension α to the size of dispersed phase R, α/R, was obtained for 80/20 and 60/40 PMMA/SAN blends, and the two different morphology were also observed.     

Impact spreading and absorption of Newtonian droplets on topographically irregular porous materials

This paper concerns the combined influences of high impaction rates and surface geometries on the spreading-absorption behaviour of Newtonian fluid droplets under non-equilibrium conditions. A volume of fluid method is used to simulate the motion of up to diameter droplets onto surfaces comprising protuberances below in width. On both flat and systematically arranged pyramidal surfaces, spreading is an exponential function of the maximum depth of absorption. Surface spreading is furthermore a linear function of the droplet Reynolds number and a simple method of predicting spreading from the Reynolds number has been identified herein. At high impact velocities, kinetic energy driven spreading dominates over wetting driven flow; though, its relativity to absorption is a function of the droplet pressure pulse. Lateral (plan view) spreading on irregular surfaces is found to be inversely proportional to surface roughness. In this paper, surface geometry is shown to be a critical determinant of both spreading and absorption and thereby, the research herein provides details that could be useful in functionalising surfaces. Finally, new surface roughness equations derived herein make the comparison of surface spreading to surface roughness more realistic than commonly used roughness factors such as the Wenzel and R_a roughness.     

Fracture energy-fracture stress relationship for weak polymer-polymer interfaces

The bulk thick films of high-molecular-weight atactic polystyrene (PS) were brought into contact at a small contact pressure ≤0.2 MPa at a constant healing temperature T_h below the calorimetric glass transition temperature of the bulk . Fracture energy G and fracture stress σ of the auto-adhesive joints PS-PS were measured at ambient temperature in the T-peel test and the lap-shear joint geometry, respectively. In the framework of the diffusion controlled mechanism of the development of these two mechanical properties suggesting their evolution as and (t_h is the healing time), and as G∝1/T_h and σ∝1/T_h, a linear relationship between G^1/2 and σ, valid over a temperature range of , has been found. The penetration depth of 0.5 nm corresponding to the value of G calculated using the measured value of σ developed at for 24 h was reasonably smaller than the thickness of the surface mobile layer of 1 nm predicted by Wool's rigidity percolation theory for thick bulk PS films. The feasibility of a full healing of polymer-polymer interfaces below has been discussed. The dependence of an apparent activation energy characterising the process of segmental motions at PS surfaces and interfaces on the approach and the physical property chosen for its calculation has been analysed.     

Homogeneous crystal nucleation in droplets as a method for determining the line tension of a crystal-liquid-vapor contact

The line tension of a three-phase contact is believed to play an important role in phase transition and phase equilibria in multiphase nanoscale systems hence the need in developing various methods for its experimental evaluation. In this paper we suggest an indirect experimental method for determining the line tension of a solid-liquid-vapor contact based on experiments on homogeneous crystallization of droplets. The underlying idea explores our recent finding that the line tension can give rise to an important contribution to the free energy of formation of a crystal nucleus in a surface-stimulated mode when one of its facets forms at the droplet surface and thus represents a “crystal-vapor” interface. The proposed method requires experimental data on the rate of homogeneous crystal nucleation as a function of droplet size. However, it can provide a rough estimate of line tension even if the rate is known only for one droplet size. Using the method to examine experimental data on homogeneous crystal nucleation in droplets of 19, 49, and radii at , we evaluated the line tension of ice-(liquid)water-(water)vapor contact to be positive and of the order of consistent with the current expectations.     

Effects of shear flow on a semidilute polymer solution under phase-separating condition

We studied the effects of a step-up shear flow from zero shear rate to the given shear rate, , on formation of shear-induced structures for a semidilute polystyrene (PS)/diethyl malonate (DEM) solution below its cloud point temperature where the solution undergoes phase separation via spinodal decomposition (SD) in quiescent state. We elucidated that the effects of step-up shear can be divided into two regions: below and above a critical shear rate, . At , growing phase-separated domains via SD are found to be deformed under the flow, so that FFT spectra of the shear-microscopy images become elliptical with the wave number q_mx at the maximum intensity parallel to the flow being smaller than the corresponding wave number q_mz parallel to the neutral axis. However, strikingly enough, the aspect ratio q_mz/q_mx of the elliptic spinodal ring observed for this system was much smaller than that observed for binary fluids. The unique feature was proposed to be the elastic effect inherent in this system. When is larger than , however, initially phase-separating structures via SD are strongly deformed and distorted. Interestingly enough, the light scattering pattern was transformed from the isotropic ring pattern into the butterfly pattern. This is interpreted as follows: when , there may not be enough time for the domains composed of elastically deformed swollen-network chains to relax, and consequently the domains are cooperatively disrupted. The disrupted domains tend to squeeze solvent in order to release the elastic free energy stored in the deformed swollen-network chains, resulting in anisotropic domain more extended to neutral axis than flow direction and hence giving rise to the butterfly pattern.     

A kinetic study on anaerobic reduction of sulphate, part II: incorporation of temperature effects in the kinetic model

The effects of temperature on the kinetics of anaerobic sulphate reduction were studied in continuous bioreactors using acetate as an electron donor. Across the range of temperatures applied from 20 to , the increasing of volumetric loading rate up to 0.08 to resulted in a linear increase in reduction rate of sulphate. The increasing reaction rate showed a lower dependence on volumetric loading rate in the range 0.1-. Further increase in volumetric loading rate above was accompanied by wash out of bacterial cells and a sharp decrease in reaction rate. Despite a similar pattern for dependency of reaction rate on volumetric loading at all temperatures tested, the magnitude of reaction rate was influenced by temperature, with a maximum rate of observed at . The effect of temperature on maximum specific growth rate (μ_max) and bacterial yield was insignificant. The values of maximum specific growth rate and yield were and 0.56-0.60 kg bacteria (), respectively. The decay coefficient (k_d) and apparent saturation constant () were both temperature dependent. The increase of temperature resulted in decreased values of , and higher values for k_d. Using the experimental data effect of temperature was incorporated in a kinetic model previously developed for anaerobic reduction of sulphate.     

Droplet impact and spreading: Droplet formulation effects

This work investigates a food application, where 2.8 mm droplets containing maltodextrin DE5 at 20 and 40 wt% in water, impact a smooth anhydrous milkfat surface at 1.7 to velocities. Results show that greater maximum spread diameters are achieved with the higher impact velocities and lower viscosity droplets. Incorporating surfactants to lower the liquid-vapour interfacial tension (from 72 to did not affect the maximum spread diameter but significantly limited droplet recoil, giving final spreading extents that cover close to three times the surface area compared to droplets without the surfactant. From these results, simple formulation and operating guidelines are proposed to achieve a maximum final droplet spread diameter for air-suspension particle coating operations.     

New approach to the formulation of hydrogel beads by emulsification/thermal gelation using a static mixer

A new approach in the formulation of hydrogel beads by emulsification/in situ thermal gelation using static mixer technology is described. κ-Carrageenan was selected as the model hydrogel. The emulsion generated by a Sulzer SMX6 static mixer consisted of warm κ-carrageenan sol (1.5% w/w in water or ) as the dispersed phase, and ambient temperature sunflower seed oil as the continuous phase. Dispersion followed by in situ gelation of κ-carrageenan droplets was possible within a short residence time (1-) in the static mixer, under defined operational conditions, known as the feasibility region. This region was defined as the zone of operation conditions necessary to obtain discrete gel beads, within a defined range of κ-carrageenan solution injection temperature, volume fraction and total flowrate. The temperature boundaries of the feasibility region were determined by the κ-carrageenan gelation temperature and solution viscosity. The resulting beads had a Sauter mean diameter ranging from 350 to , which decreased with the increase of κ-carrageenan injection temperature, total flowrate and/or the number of static mixer elements. Theoretical values of maximal bead diameter and Sauter mean diameter were calculated on the base of critical Weber number, which was demonstrated through good agreement with the experimental values. It was demonstrated that an existing model for the prediction of gel bead diameter in a SMX static mixer is applicable for the new procedure described in this study.     

Comments to “Analysis of constant rate period of spray drying of slurry” by Liang et al., 2001

In the study by Liang et al. [2001. Analysis of constant rate period of spray drying of slurry. Chemical Engineering Science 56, 2205-2213] the Darcy flow of liquid through a pore system of primary particles to the surface of a slurry droplet was applied for the constant rate period. Steep primary particle concentration gradients inside droplets with a primary particle size of were observed. Unfortunately, the boundary condition at the droplet surface for the parabolic second-order PDE did not conserve the solid mass in the droplet, and the plots for the primary particle concentration profiles in the droplets were incorrect. In this letter we derive the correct boundary condition equation. Furthermore, we show that the primary particle concentration profiles inside the droplets are flat when the primary particles have a size of . We conclude that the model presented by Liang et al. is unable to predict the formation of hollow particles.     

Dispersion mechanism of nano-particulate aggregates using a high pressure wet-type jet mill

A high pressure wet-type jet mill was employed to disperse nano-particle suspensions. Commercially available nano-particles, fumed silica (SiO₂) of primary particle diameter (d₀) ranging from 7 to 40 nm, alumina (Al₂O₃) of and titanium oxide (TiO₂) of were dispersed in the continuous phase up to viscosity . Ion exchanged water, aqueous ethylene glycol and aqueous polyethylene glycol solutions with molecular weight up to 2 000 000, were used as the continuous phase. Particle size distribution, zeta potential and suspension viscosity were measured under a wide range of process conditions. The smaller the d₀ was, the harder it was to disperse the aggregates. Zeta potential was largely dependent on d₀ at any process conditions and became dependent on η_c for . The energy barrier was evaluated by taking van der Waals attractive forces, electrostatic repulsive forces and dispersive forces into consideration. Cavitation measurements showed a negligible cavitation during the passage through the jet mill; therefore aggregate disruption was modeled for fully turbulent flow. Aggregate disruption occurred in inertia sub-range for and in viscous sub-range for . By balancing mechanical energy with turbulent disruptive energy, a mechanistic model was developed for each sub-range. The analysis of fractal dimensionality showed that nano-aggregates were made up by particle-particle collision in inertia sub-range and orthokinetic cluster-cluster collision in viscous sub-range. The rheological data obtained were expressed according to a modified Casson model.     

Synthesis, characterization and thermodynamic properties of poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone)

Poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone) P(MHPMA-co-VP) was synthesized in 1, 4-dioxane solution using benzoyl peroxide (BPO) as initiator at 60 °C. The copolymer was characterized by ¹H ¹³C NMR, FT-IR, DSC, TGA, size exclusion chromatography analysis (SEC) and elemental analysis techniques. According to SEC, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of PMHPMA-co-VP were found to be 58,000, 481,000 g/mol and 8.26, respectively. According to TGA, carbonaceous residue value of PMHPMA-co-VP was found to be 6% at 500 °C. Also, some thermodynamic properties of PMHPMA-co-VP such as the adsorption enthalpy, ΔH_a, molar evaporation enthalpy, ΔH_v, the sorption enthalpy, , sorption free energy, , sorption entropy, , the partial molar free energy, , the partial molar heat of mixing, , at infinite dilution was determined for the interactions of PMHPMA-co-VP with selected alcohols and alkanes by inverse gas chromatography (IGC) method in the temperature range of 323-463 K. According to the specific retention volumes, , the weight fraction activity coefficients of solute probes at infinite dilution, , and Flory-Huggins interaction parameters, between PMHPMA-co-VP-solvents were determined in 413-453 K. According to and , selected alcohols and alkanes were found to be non-solvent for PMHPMA-co-VP at 413-453 K. The glass transition temperature, T_g, of the PMHPMA-co-VP found to be 370 and 363 K, respectively, by IGC and DSC techniques, respectively.     

Aspirin particle formation by electric-field-assisted release of droplets

An aspirin solution flowing through a needle at () was subjected to an electric field of to generate droplets. The aspirin droplet size distribution generated was measured and is shown to be trimodal with the most significant size, . Deposited droplets and their aspirin crystal relics were also studied by electron microscopy. Immediately after deposition, the droplets were remarkably uniform in size and produced particulate crystals and were considerably more regular in morphology when compared with those formed without using an electric field.     

Modeling of wet jet in fluidized bed

A wet jet zone is established in many applications wherever feeding and dispersing a liquid, solution or slurry into fluidized bed by gases is needed. In the present study, a simple mathematical model has been developed to simulate the wet jet in fluidized bed. The different stages involved inside the jet zone have been estimated and analyzed.The evaporation stage of traveling droplets through the jet flare has been treated. The rates of evaporation of each size at all positions along the jet flare have been estimated according to the velocities and surrounding conditions. The final droplet sizes have been determined. Moreover, the total evaporation rate from traveling droplets, before collision either with entrained sand particles or flare boundaries, has been estimated. The traveling droplets, partially evaporated, may collide and settle on entrained sand particles. The model predicts the settlement rates of liquid droplets on entrained sand particles. The total part evaporated from settled liquid has been estimated as well.The study has been applied to the pneumatic feeding of liquid fuel into fluidized bed combustors operating at . The model has been utilized to predict the ratio of fuel vapor that releases inside the jet flare. The remaining part is assumed to evaporate inside the emulsion phase. Three different liquid fuels have been considered: a heavy oil, diesel fuel and gasoline. The main independent variables are those related to the injection conditions including the initial velocity of dispersing air, u₀, and air-to-liquid mass ratio, ALR.The model results demonstrate that only very small droplets completely evaporate inside the flare. The liquid settling over the entrained sand particles plays an essential role in the fuel evaporation inside the flare. The phenomenon is dominant at conditions that result in generation of droplets of larger sizes, i.e., heavier fuel, lower u₀, and greater ALR. The ratio of vapor fuel released in jet flare increases with lighter fuel, higher u₀ and lower ALR. At and ALR=1.0 nearly all-liquid fuel evaporates inside the flare.     

Proteolytic cleaning of a surface-bound rubisco protein stain

We present a combined experimental and mathematical study of the proteolysis of a surface-bound rubisco protein stain. The adsorption and desorption of subtilisin A (SA) onto and from surface-bound rubisco films were found to be a strong function of the surface chemistry underlying the protein stain; the stain acted as a biosensor able to convey information about the underlying surface to the attacking protease. The apparent protease adsorption rate constants (k_a) were 0.016±0.007, 0.014±0.004 and while the apparent desorption rate constants (k_d) were 1.60±0.15, 1.05±0.02 and for hydrophobic, neutral-hydrophilic and negatively charged hydrophilic surfaces, respectively. The apparent proteolysis rate constant of surface-bound rubisco and the enzyme deactivation rate constant were estimated to be and , respectively, independent of underlying surface chemistry. The results demonstrated higher protein removal from the charged hydrophilic surface relative to the other two surfaces. Rubisco cleanability from the charged and hydrophobic surfaces increased with increasing bulk enzyme concentration (and hence surface enzyme concentration) and was better for the charged surface, perhaps reflecting the higher k_a value. Conversely, rubisco cleanability from the neutral hydrophilic surface was surprisingly insensitive to variation in bulk enzyme concentration. Overall cleaning efficiency was also substantially lower for the neutral hydrophilic surface when compared with the hydrophobic surface, even though k_a values for each surface were similar. These findings indicate that surface proteolysis is significantly impaired at low values of k_d, suggesting that enzyme mobility at the interface may be closely linked to cleaning performance. The model presented here is expected to be a useful tool in the detergent industries to screen and gauge the cleaning performance of detergent-enzyme formulations, and may also be able to facilitate the design of surface treatments that convey cleaning signals to attacking proteases.     

Thermal denaturation and folding rates of single domain proteins: size matters

We analyze the dependence of thermal denaturation transition and folding rates of globular proteins on the number of amino acid residues, N. Using lattice Go models we show that ΔT/T_F∼N⁻¹, where T_F is the folding transition temperature and ΔT is the transition width computed using the temperature dependence of the order parameter that distinguishes between the unfolded state and the native basin of attraction. This finding is consistent with finite size effects expected for the systems undergoing a phase transition from a disordered to an ordered phase. The dependence of the folding rates on N for lattice models and the dataset of 57 proteins and peptides shows that with 0<β≤2/3 provides a good fit, where C is a β-dependent constant. We find that with an average (over the dataset of proteins) can estimate optimal protein folding rates, to within an order of magnitude in most cases. By using this fit for a set of proteins with β-sheet topology we find that k_F⁰≈k_U⁰, the prefactor for unfolding. The maximum ratio of k_U⁰/k_F⁰≈10 for this class of proteins.