Serguei R. Kosvintsev

Spreading of liquid drops over porous substrates.

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) has been investigated in the case of both complete wetting (silicone oils of different viscosities) and partial wetting (aqueous SDS solutions of different concentrations). Nitrocellulose membranes of different porosity and different average pore size have been used as a model of thin porous layers, glass and metal filters have been used as a model of thick porous substrates. The first problem under investigation has been the spreading of small liquid drops over thin porous layers saturated with the same liquid. An evolution equation describing the drop spreading has been deduced, which showed that both an effective lubrication and the liquid exchange between the drop and the porous substrates are equally important. Spreading of silicone oils over different nitrocellulose microfiltration membranes was carried out. The experimental laws of the radius of spreading on time confirmed the theory predictions. The spreading of small liquid drops over thin dry porous layers has also been investigated from both theoretical and experimental points of view. The drop motion over a dry porous layer appears caused by the interplay of two processes: (a). the spreading of the drop over already saturated parts of the porous layer, which results in a growth of the drop base, and (b). the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and a growth of the wetted region inside the porous layer. As a result of these two competing processes the radius of the drop base goes through a maximum as time proceeds. A system of two differential equations has been derived to describe the time evolution of the radii of both the drop base and the wetted region inside the porous layer. This system includes two parameters, one accounts for the effective lubrication coefficient of the liquid over the wetted porous substrate, and the other is a combination of permeability and effective capillary pressure inside the porous layer. Two additional experiments were used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters were determined. Experiments were carried out on the spreading of silicone oil drops over various dry nitrocellulose microfiltration membranes (permeable in both normal and tangential directions). The time evolution of the radii of both the drop base and the wetted region inside the porous layer was monitored. In agreement with our theory all experimental data fell on two universal curves if appropriate scales were used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer using a dimensionless time scale. Theory predicts that (a). the dynamic contact angle dependence on the dimensionless time should be a universal function, (b). the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in our system. These predictions are in the good agreement with our experimental observations. In the case of spreading of liquid drops over thick porous substrates (complete wetting) the spreading process goes in two similar stages as in the case of thin porous substrates. In this case also both the drop base and the radii of the wetted area on the surface of the porous substrates were monitored. Spreading of oil drops (with a wide range of viscosities) on dry porous substrates having similar porosity and average pore size shows universal behavior as in the case of thin porous substrates. However, the spreading behavior on porous substrates having different average pore sizes deviates from the universal behavior. Yet, even in this case the dynamic contact angle remains constant over the duration of the second stage of spreading as in the case of spreading on thin porous substrates. Finally, experimental observations of the spreading of aqueous SDS solution over nitrocellulose membranes were carried out (case of partial wetting). The time evolution of the radii of both the drop base and the wetted area inside the porous substrate was monitored. The total duration of the spreading process was subdivided into three stages: in the first stage the drop base growths until a maximum value is reached. The contact angle rapidly decreases during this stage; in the second stage the radius of the drop base remains constant and the contact angle decreases linearly with time; finally in the third stage the drop base shrinks while the contact angle remains constant. The wetted area inside the porous substrate expands during the whole spreading process. Appropriate scales were used to have a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate, and the dynamic contact angle vs. the dimensionless time. Our experimental data show: the overall time of the spreading of drops of SDS solutions over dry thin porous substrates decreases with the increase of surfactant concentration; the difference between advancing and hydrodynamic receding contact angles decreases with the surfactant concentration increase; the constancy of the contact angle during the third stage of spreading has nothing to do with the hysteresis of contact angle, but determined by the hydrodynamics. Using independent spreading experiments of the same drops on a non-porous nitrocellulose substrate we have shown that the static receding contact angle is equal to zero, which supports our conclusion on the hydrodynamic nature of the hydrodynamic receding contact angle on porous substrates.

Pore design and engineering for filters and membranes

In filtration, the concept of pore size is not easy to define. In microfiltration, there are numerous advantages in employing a surface filtering membrane, rather than one relying on depth filtration mechanisms from a tortuous pore flow channel. Modern manufacturing techniques provide means to produce surface filtering membranes. For filtration, it is shown that a suitable pore design is an array of long thin slots. An analysis of fluid flow through the slots suggests that a short slot is adequate, but experimental data with suspended material indicates that slot length is important. Using long slots and careful control of the flow through the membrane it is possible to filter deforming particles such as oil drops from water.

PLGA particle production for water-soluble drug encapsulation: degradation and release behaviour.

Particles for subcutaneous depot use encapsulating a model water-soluble drug have been produced from poly(lactic-glycolic acid) (PLGA) using a membrane emulsification-solvent evaporation technique. The release behaviour, mainly the change in size and inner morphology are reported. During release, the particles initially swelled in size, then reduced. A diffusion based model, taking in to account the change in particle size, is presented. Surface erosion is evident from the particle size and image evidence, and the diffusion model provides a fit to the data even during the surface erosion period, suggesting that the model drug diffuses before the particle degrades.

Rheological behaviour of solutions of poly(2-hydroxyethyl methacrylamide) in glycerine

Abstract The rheological behaviour of dilute and semidilute solutions of poly(2-hydroxyethyl methacrylamide) in glycerine is described, emphasising the study of the variation of both the zero shear rate viscosity, η0, and the steady-state compliance function, Je0, with concentration. Shear rate fields, above a critical value, may promote molecular tight ‘structures’ that enhance the viscosity of the semidilute solutions and, as a result, these systems exhibit shear-thickening behaviour. However, the dilute solutions display shear-thinning behaviour and obey the Cox–Merz rule. The flow transients appearing after either application of a constant shear rate or cessation of flow were studied for semidilute solutions. The shear-thickening behaviour of these solutions is interpreted in terms of the formation of hydrogen bonds between the acrylamide groups and the carbonyl groups of different chains.

Low Pressure Microfilter Design Aspects and Filtration Performance

Abstract A microfilter should retain micron-sized material yet provide minimal resistance to liquid flow. A slotted pore surface microfilter was oscillated while filtering yeast cells under constant rate. At shear rates over 7760 s−1, a pore blocking model fitted the data. The operating pressure was very low (<1000 Pa), but particle retention was limited by the 4 micron pore slot width. A sintered glass micro-bead coating improved yeast rejection: 95% at 1.7 microns at a shear rate of 5000 s−1, with a 1.2 kPa transmembrane pressure. Two models were validated to assist with the design of future micro-bead coatings constructed from spherical particles.

Slotted pore microfilters for oil/water filtration, fractionation and aggressive filtration environments

Abstract In the UK, researchers have been investigating ways of separating oil and other particles from water. They have developed and designed a new filter that uses microfiltration media that are similar to very fine sieves. Furthermore, there is no internal deposition of solids, and the pressure that is required to pass liquids through the media is low. This article provides details of this technology, which could be used on offshore oil rigs to vastly reduce the amount of oil discharged into the worlds oceans.

Electric field effects on the stability of a thermogravitational flow in a vertical capacitor

Abstract The effect of electrization of a poorly conducting liquid and the action of an electric field on the stability of the base flow in a vertical layer with un-equal but constant temperatures at its vertical boundaries was investigated numerically and experimentally using a linear dependence of conductivity on temperature. By solving the appropriate EHD and heat equations we have found that for liquids of low electrical Prandtl number, P e , and low values of electrical Grashoff number, G e , an electric field stabilizes the base flow thus leading to a higher critical value of the thermal Grashoff number G for thermoconvective instability. However as G e increases, stabilization is replaced by destabilization and the critical G for the onset of instability decreases to zero as G e increases. We have been able to observe oscillatory electro-thermoconvective motions in a wide range of values of the parameters although for low enough values of G e the electroconvective instability leads to steady-convection in the form of rolls provided the values of G are low enough.