Guoping Lian

Discrete particle simulation of agglomerate impact coalescence

Abstract This paper describes computer simulations of pendular state wet agglomerates undergoing pair-wise collisions. The simulation method is based upon a ‘soft’ discrete particle formulation. Each agglomerate comprised 1000 primary particles with the interparticle interactions modelled as the combination of the solid–solid contact forces and also the forces developed at discrete liquid bridges between neighbouring particles. For the range of collisional velocities implemented, the agglomerates invariably coalesced. The energy dissipated was associated primarily with the viscous resistance of the fluid and the interparticle friction rather than by liquid bridge bond rupture. The structure of the resultant coalesced agglomerate was highly disordered and depended on the impact velocity. As the impact velocity approached zero, the agglomerates behaved like two rigid bodies bonded together. When the impact velocity was increased, the size of the circumscribing sphere of the coalesced agglomerate decreased and reached a minimum value at a critical velocity above which an increase in the circumscribing sphere size occurred due to extensive flattening. An increase in the viscosity of the interstitial fluid resulted in an increase in the proportion of energy dissipated by viscous resistance and a decrease in the proportion dissipated due to interparticle friction. An increase in the fluid viscosity also resulted in an increase in the critical impact velocity at which the size of the circumscribing sphere of the coalesced agglomerate was a minimum.

Elastohydrodynamic collisions of solid spheres

Recent developments in solving the problem of the elastohydrodynamic collision between two solid elastic bodies involved elaborate numerical procedures in order to simultaneously account for the elastic deformation of the solid surfaces and viscous fluid pressure. This paper describes a simple analytical approximation based upon a Hertzian-like profile for the elastic deformation of the two solid elastic spheres. By introducing a scaling coefficient, a closed-form solution has been developed which is capable of predicting the evolution of the relative particle velocity, force and restitution coefficient to an accuracy that is comparable with the exact numerical solutions.

Recent advances in predicting skin permeability of hydrophilic solutes

Understanding the permeation of hydrophilic molecules is of relevance to many applications including transdermal drug delivery, skin care as well as risk assessment of occupational, environmental, or consumer exposure. This paper reviews recent advances in modeling skin permeability of hydrophilic solutes, including quantitative structure-permeability relationships (QSPR) and mechanistic models. A dataset of measured human skin permeability of hydrophilic and low hydrophobic solutes has been compiled. Generally statistically derived QSPR models under-estimate skin permeability of hydrophilic solutes. On the other hand, including additional aqueous pathway is necessary for mechanistic models to improve the prediction of skin permeability of hydrophilic solutes, especially for highly hydrophilic solutes. A consensus yet has to be reached as to how the aqueous pathway should be modeled. Nevertheless it is shown that the contribution of aqueous pathway can constitute to more than 95% of the overall skin permeability. Finally, future prospects and needs in improving the prediction of skin permeability of hydrophilic solutes are discussed.

On the squeeze flow of a power-law fluid between rigid spheres

The lubrication solution for the squeeze flow of a power-law fluid between two rigid spherical particles has been investigated. It is shown that the radial pressure distribution converges to zero within the gap between the particles for any value of the flow index, n, provided that the gap separation distance is sufficiently small. However, in the case of the viscous force, it is useful to consider that there are two contributions. The first is developed in the inner region of the gap and corresponds to the lubrication limit. The second is due to an integration of the pressure in the adjacent outer region of the gap. The relative contribution to the force in this outer region increases as n decreases and the separation distance increases. In particular, for flow indices in the range n>1/3, the contribution in the outer region is negligible if the separation distance is sufficiently small. For n⪯1/3, this is the dominant term and an accurate prediction of the viscous force is possible only for discrete liquid bridges. Based on “zero” pressure and lubrication criteria for the upper limits of integration, two closed-form solutions have been derived for the viscous force. Both are accurate for n>0.5 and are in close agreement with a previously published asymptotic solution in the range n>0.6. For smaller values of n, the asymptotic solution over-estimates the viscous force and predicts a singularity when n approaches 1/3. The two closed-form solutions show continuous and monotonic behaviour for all values of n. Moreover, the solution satisfying the lubrication limit is valid in the range n<1/3 provided that it is restricted to liquid bridges.

Effects of material properties on granular flow in a silo using DEM simulation

In order to test the effect of material properties on flowability of particulate materials, discharge procedures of spherical particles within a flat-bottomed model silo with three sets of material properties, i.e., soft and hard without adhesion and adhesive hard, were simulated using the Discrete Element Method. For each system, three particles on the center line were selected and their instant vertical velocity components were traced. In addition, both discharge and the rate were recorded throughout the procedure. The predicted results show that, for both the systems without adhesion, though the soft has a material modulus only 1/1000 of the hard, there are no significant differences in f low pattern and discharge rate. This suggests that a soft system can be used to predict the behavior of a hard one to save CPU time in a gravity-driven granular flow. On the other hand, comparison between both hard systems shows that adhesion can significantly reduce the flowability in granular flow. By analyzing the velocity plot for the traced particles, free fall was clearly detected above the decompression zone, indicating the motion of a particle in a granular flow can be resolved as free fall together with the movement due to particle collision. In addition, select dynamic behavior related to the kinetic fluctuations affecting flow was observed. discrete element method silo granule flow particulate material

Prediction of human skin permeability using artificial neural network (ANN) modeling.

AbstractAim:To develop an artificial neural network (ANN) model for predicting skin permeability (log Kp) of new chemical entities.Methods:A large dataset of 215 experimental data points was compiled from the literature. The dataset was subdivided into 5 subsets and 4 of them were used to train and validate an ANN model. The same 4 datasets were also used to build a multiple linear regression (MLR) model. The remaining dataset was then used to test the 2 models. Abraham descriptors were employed as inputs into the 2 models. Model predictions were compared with the experimental results. In addition, the relationship between log Kp and Abraham descriptors were investigated.Results:The regression results of the MLR model were n=215, determination coefficient (R2)=0.699, mean square error (MSE)=0.243, and F=493.556. The ANN model gave improved results with n=215, R2=0.832, MSE=0.136, and F=1050.653. The ANN model suggests that the relationship between log Kp and Abraham descriptors is non-linear.Conclusion:The study suggests that Abraham descriptors may be used to predict skin permeability, and the ANN model gives improved prediction of skin permeability.

COUPLED HEAT AND MOISTURE TRANSFER DURING MICROWAVE VACUUM DRYING

A general mechanistic model has been developed to describe the coupled heat and moisture transfer during dielectrically-assisted drying of porous materials. The model considers the moisture transfer as the combination of the liquid water and vapor water transfer that takes place simultaneously. The heat transfer is based on the energy conservation of the sensible heat, latent heat and source heat of microwave power. The derived heat and moisture transfer equations are fully coupled. When the temperature gradient induced moisture transfer is sufficiently small, the moisture transfer equation degenerates to the isothermal equation. By using the finite element method, the fully coupled equations have been solved numerically and applied to simulate the heat and moisture transfer during microwave vacuum drying of a concentrated water soluble food paste. The predicted drying rate is in good agreement with the experimental data.

Non-animal models of epithelial barriers (skin, intestine and lung) in research, industrial applications and regulatory toxicology

Models of the outer epithelia of the human body - namely the skin, the intestine and the lung - have found valid applications in both research and industrial settings as attractive alternatives to animal testing. A variety of approaches to model these barriers are currently employed in such fields, ranging from the utilization of ex vivo tissue to reconstructed in vitro models, and further to chip-based technologies, synthetic membrane systems and, of increasing current interest, in silico modeling approaches. An international group of experts in the field of epithelial barriers was convened from academia, industry and regulatory bodies to present both the current state of the art of non-animal models of the skin, intestinal and pulmonary barriers in their various fields of application, and to discuss research-based, industry-driven and regulatory-relevant future directions for both the development of new models and the refinement of existing test methods. Issues of model relevance and preference, validation and standardization, acceptance, and the need for simplicity versus complexity were focal themes of the discussions. The outcomes of workshop presentations and discussions, in relation to both current status and future directions in the utilization and development of epithelial barrier models, are presented by the attending experts in the current report.

Determination of partition and binding properties of solutes to stratum corneum

The binding property of a number of relatively hydrophilic solutes to native and delipidized stratum corneum (SC) and their partition coefficients to extracted lipid have been measured by equilibration experiments to expand the current database which consisted of mostly hydrophobic solutes. Using the extended database, quantitative structure property relationships (QSPR) have been proposed for predicting the partition and binding coefficients of both hydrophobic and hydrophilic solutes to the SC protein, and lipid. Solute partition to the SC lipid is best fitted by PC(lip/w)=K(ow)(0.69) and solute binding to the SC protein is best described by PC(pro/w)=4.2K(ow)(0.31). The two QSPR models of solute partition to the SC lipid and binding to the SC protein have been further combined into a two-phase model to predict the overall partition coefficient of solutes to the stratum corneum (K(sc/w)). Our study not only extends the database of solute partition and binding properties of the SC to include hydrophilic solutes, but also demonstrates that the thermodynamic equilibrium properties of the SC partition and binding can be fitted with good accuracy by combining QSPR models with the multiphase and heterogeneous structures of the SC.

Experimental and theoretical studies on the binding of epigallocatechin gallate to purified porcine gastric mucin.

Binding of epigallocatechin gallate (EGCG) to highly purified short side-chain porcine gastric mucin similar to human MUC6 type has been studied by ultraviolet-visible absorption spectroscopy (UV-vis), ultrafiltration isothermal titration microcalorimetry (ITC) and transmission electron microscopy (TEM). The thermodynamic equilibrium of EGCG binding to mucin has been quantitatively determined using ultrafiltration and high-performance liquid chromatography (HPLC)-UV/vis. The relationship suggests multilayer binding rather than simple Langmuir monolayer binding of EGCG. By combining the ultrafiltration and ITC data, the thermodynamic parameters of EGCG binding to mucin have been obtained. The binding constant for the first layer is about an order of magnitude higher than that of the consecutive multilayers. Negative entropy indicates multilayer of EGCG formed. Hydrogen bonding may be responsible for the multilayer formation. Increasing temperature resulted in a decrease in the binding affinity, further suggesting that hydrogen bonds dominated the interaction energy. A TEM micrograph of the EGCG-mucin complex revealed a monodispersion of blobs similar to pure mucin solution but with relatively bigger size (about twice). It is proposed that the EGCG-mucin binding process occurs by single and/or cluster of EGCG molecules driven to the surface of the two hydrophobic globules of mucin by hydrophobic interaction followed by hydrogen bond interaction between EGCG and mucin. Further adsorption of EGCG molecules onto bound EGCG molecules to form multilayers can also occur. This fits well with the observations that EGCG-mucin interaction followed a multilayer adsorption isotherm, the energy released is dominated by hydrogen bonds, and no large aggregates were formed.