Weizhong Li

Molecular Dynamics Study of Effects of Temperature and Concentration on Hydrogen-Bond Abilities of Ethylene Glycol and Glycerol: Implications for Cryopreservation

The state of intracellular water is important in all phases of cryopreservation. Intracellular water can be transported out of the cell, transferred into its solid phase, or blocked by cryoprotectants and proteins in the cytoplasm. The purpose of the present study is to determine the amount of hydrogen-bonded water in aqueous ethylene glycol and glycerol solutions. The effects of temperature and concentration on the density and the hydrogen bonding characteristics of the solution are evaluated quantitatively in this study. To achieve these aims, a series of molecular dynamics simulations of ethylene glycol/water and glycerol/water mixtures of molalities ranging from 1 to 5 m are conducted at 1 atm and at 273, 285, and 298 K, respectively. The simulation results show that temperature and concentration have variable effects on solution density. The proportion of the hydrogen-bonded water by solute molecules increases with rising molality. The ability of the solute molecules to hydrogen bond with water molecules weakens as the solution becomes more concentrated. Moreover, it turns out that the solution concentration can influence the hydrogen bonding characteristics more greatly than the temperature. The glycerol molecule should be a stronger water blocker than the ethylene glycol molecule corresponding to the same conditions. These findings provide insight into the cryoprotective mechanisms of ethylene glycol and glycerol in aqueous solutions, which will confer benefits on the cryopreservation.

An alternating dependent variables (ADV) method for treating slip boundary conditions of free surface flows with heat and mass transfer

An alternating dependent variables (ADV) method is proposed to treat slip boundary conditions for interfacial flows of a nonspherical bubble in liquid when Cartesian velocity components are chosen as dependent variables in momentum equations written in nonorthogonal body-fitted coordinates. Cartesian interfacial velocities are solved alternatively at different segments of bubble profile, hence numerical instabilities due to a nearly infinite and zero slop of the profile are avoided. Numerical results indicate that the ADV is a suitable method for solving free surface flows of nonspherical bubbles with large curvature. On this basis, the interfacial characteristics of three types of nonspherical bubbles are presented.

Direct-predictor method for solving steady terminal shape of a gas bubble rising through a quiescent liquid

A direct-predictor method is proposed to predict a steady terminal shape of a rising bubble through a quiescent liquid, in which a local imbalance of the total normal stress is used to improve new positions of the bubble interface by an iteration approach. The deformation of a nearly skirted bubble is tracked by applying this method in nonorthogonal boundary-fitted coordinates (BFCs). Completed and satisfactory solutions are achieved for 0.5 h Re h 200 and We h 50 . The results show that the method is very effective for solving such free surface flows past a bubble with larger deformation.

Two applications of the thermogram of the alcohol/water binary system with compositions of cryobiological interests.

Quantitative analyses of the bound water content in the alcohol aqueous solution and its osmotic behavior should be cryobiologically significant. This paper has presented two applications of the thermogram of the alcohol/water system recorded by differential scanning calorimeter (DSC). Both applications are: (1) generating the quantitative relationship between the bound water content and the solution composition; (2) calculating the osmotic virial coefficients for alcohols. Five alcohols including methanol, ethanol, ethylene glycol, propylene glycol and glycerol are investigated. In the present study, partial binary phase diagrams of these five alcohol solutions are determined in the first place. The bound water contents in these solutions are quantitatively evaluated by three criteria afterwards. In the end, the osmotic virial coefficients for these alcohols are calculated according to the osmotic virial equation. It is turned out that the bound water fraction out of the total water content increases with a rising molality. The ability of the solute to restrict water molecules can be weakened when the solution becomes more concentrated. The results also indicate that propylene glycol should be the strongest water-blocker while methanol the weakest one. These findings can deepen our understanding of the cryoprotective properties of the alcohols from the perspectives of their roles in binding free water and promoting the osmotic efflux of cell water.

Kinetics of osmotic water flow across cell membranes in non-ideal solutions during freezing and thawing

Cryopreservation requires quantitatively analytical models to simulate the biophysical responses of biomaterials during cryopreservation. The Mazur model and other improved ones, such as Karlsson model concerning solutions containing cryoprotectants (CPA), are somehow precluded by some minor points, particularly, the assumption of ideal solutions. To avoid the ideal solution assumption, in this study a new method is developed to simulate water transport across cell membranes in non-ideal solutions during cooling and thawing. The comparison between osmolalities calculated by the linear freezing-point depression used in this new method and other non-ideal ones is conducted and a good agreement is achieved. In addition, in an ideal case, besides a theoretical agreement, this new approach has been validated by its numerical simulation results. Comparisons between this new approach and the traditional ones with an ideal solution assumption have been conducted based on a spherical hypothetical cell. The main results are (1) the predicted non-ideal intracellular water content is larger than the ideal results; (2) the concentration of CPA solutions is directly proportional to the deviation between the non-ideal and ideal curves. In the end, this study presents a direct description of the degree of subcooling of the protoplasm during dynamic cooling. This study demonstrates that our experimental data-based method is a valid one with clear physical interpretations, convenient expressions and a more extensive application room than traditional ones.

Kinetics of coupling water and cryoprotectant transport across cell membranes and applications to cryopreservation.

Thermodynamic and kinetic models can provide a wealth of information on the physical response of living cells and tissues experiencing cryopreservation procedures. Both isothermal and nonisothermal models have been proposed so far, accompanied by experimental verification and cryoapplications. But the cryoprotective solution is usually assumed to be dilute and ideal in the models proposed in the literature. Additionally, few nonisothermal models are able to couple the transmembrane transport of water and cryoprotectant during cooling and warming of cells. To overcome these limitations, this study develops a whole new set of equations that can quantify the cotransport of water and cryoprotectant across cell membranes in the nondilute and nonideal solution during the freezing and thawing protocols. The new models proposed here can be simplified into ones consistent with the classic models if some specific assumptions are included. For cryobiological practice, they are applied to predict the volumetric change for imprinting control region (ICR) mouse spermatozoa and human corneal keratocytes in the freezing protocol. The new models can determine the intracellular concentration of cryoprotectant more precisely than others by abandoning the assumptions such as dilute and ideal solutions and nonpermeability of membranes to cryoprotectant. Further, the findings in this study will offer new insights into the physical response of cells undergoing cryopreservation.

Lattice Boltzmann simulation of a single droplet impingement and evaporation on inclined heated surface

The impingement and evaporation processes of droplet widely exist in many industrial fields such as fuel injection in combustion engines, spray drying and turbines. When a single droplet falls and impacts on an inclined hot surface under the effect of gravity, it evaporates after contacting with the surface due to the heated transfer. The inclined angle of surfaces has great effects on droplet dynamics and heat transfer. In this work, the pseudo-potential model and a thermal lattice Boltzmann model are combined to simulate the impact process and the heat transfer. Moreover, the Peng-Robinson equation of state is incorporated in the effective density function to consider the large liquid/gas density ratio. The influences of inclined angle on droplet shape and evaporation rate are obtained and analyzed. The results show that for a fixed initial velocity, when the inclined angel increases, droplet deformation is significant as the motion between droplet and the surface is strengthened and the droplet evaporation rate gets faster since the heat transfer is enhanced.