Shu-Shen Lu

Active control of phase change from supercooled water to ice by ultrasonic vibration 2. Generation of ice slurries and effect of bubble nuclei

An effective and reliable method to generate ice slurry is necessary for cold-energy storage and transport systems. First we found that ice slurries were generated in supercooled water by ultrasonic vibration. Then we experimentally studied the effect of bubble nuclei on the phase change from supercooled water to ice induced by ultrasonic vibration. Our results show that the phase change is closely related to acoustic cavitation, and the probability of the phase change increases as the total number of bubble nuclei increases. Finally, simulation results based on the cavitation-induced nucleation of solid qualitatively agree with the experimental results.

Inhibition of nucleation and growth of ice by poly(vinyl alcohol) in vitrification solution.

Control of ice formation is crucial in cryopreservation of biological substances. Successful vitrification using several additives that inhibit ice nucleation in vitrification solutions has previously been reported. Among these additives, here we focused on a synthetic polymer, poly(vinyl alcohol) (PVA), and investigated the effects of PVA on nucleation and growth of ice in 35% (w/w) aqueous 1,2-propanediol solution by using a differential scanning calorimetry (DSC) system equipped with a cryomicroscope. First, the freezing temperature of the solution was measured using the DSC system, and then the change in ice fraction in the solution during cooling was evaluated based on images obtained using the cryomicroscope, at different concentrations of PVA between 0% and 3% (w/w). Based on the ice fraction, the change in residual solution concentration during cooling was also evaluated and then plotted on the state diagram of aqueous 1,2-propanediol solution. Results indicated that, when the partially glassy and partially frozen state was intentionally allowed, the addition of PVA effectively inhibited not only ice nucleation but also ice growth in the vitrification solution. The effect of PVA on ice growth in the vitrification solution was explained based on kinetic limitations mainly due to mass transport. The interfacial kinetics also might limit ice growth in the vitrification solution only when the ice growth rate decreased below a critical value. This coincides with the fact that PVA exhibits a unique antifreeze activity in the same manner as antifreeze proteins when ice growth rate is lower than a critical value.

Microscale study of poly(vinyl alcohol) as an effective additive for inhibiting recrystallization in ice slurries

Abstract Scanning tunneling microscopy (STM) was used to investigate the surface morphology of ice crystals containing adsorbed poly(vinyl alcohol) (PVOH) molecules inside a cold room at −7.0°C. PVOH was used as a substitute for antifreeze protein (AFP) type I, which is an effective additive for making ice slurries resistant to recrystallization. The STM images revealed microscale grooves on ice crystals made from PVOH solutions, indicating that PVOH molecules significantly influence the surface structure of the ice crystal. The length of each groove was similar to that of a PVOH molecule, indicating that PVOH molecules were adsorbed on the ice crystal surfaces. The interaction force between PVOH molecules and the ice surface was discussed by assuming a molecular structure of PVOH on the ice crystal surface, and the depression of the local freezing point was analyzed based on the surface curvature of ice revealed by STM.

Glass transition behavior of the vitrification solutions containing propanediol, dimethyl sulfoxide and polyvinyl alcohol.

Knowledge of the glass transition behavior of vitrification solutions is important for research and planning of the cryopreservation of biological materials by vitrification. This brief communication shows the analysis for the glass transition and glass stability of the multi-component vitrification solutions containing propanediol (PE), dimethyl sulfoxide (Me2SO) and polyvinyl alcohol (PVA) by using differential scanning calorimetry (DSC) during the cooling and subsequent warming between 25 and -150 degrees C. The glass formation of the solutions was enhanced by introduction of PVA. Partial glass formed during cooling and the fractions of free water in the partial glass matrix increased with the increasing of PVA concentration, which caused slight decline of glass transition temperature, T(g). Exothermic peaks of devitrification were delayed and broadened, which may result from the inhibition of ice nucleation or recrystallization of PVA.

Visual Observation of African Trypanosomes During Cryopreservation

Protozoa have been widely used for the study of cryopreservation. The survival rate after cryopreservation has always received the most attention, while the cell viability during the process of freezing and thawing has been much less studied. In the present study, we report successful cryopreservation of Trypanosoma brucei, a parasitic protozoa of human and animals, using controlled-rate freezing at 5°C/min, and real-time observation of activity using a microscope differential scanning calorimeter system during the freezing and thawing process. Trehalose used as a cryoprotective agent at a concentration of 0.4 M allowed the trypanosomes to endure freezing and thawing with >89% survival rate. Results from mechanisms analysis indicate that vitrification by trehalose contributes significantly to the protection of the trypanosomes from damage at low temperature.