Wetting films of aqueous solutions of Silwet L-77 on a hydrophobic surface.

The formation of wetting films of aqueous solutions of Silwet L-77 on hydrophobic substrates takes place only at concentrations above the critical aggregation concentration (CAC). At concentrations above the critical wetting concentration (CWC) a new phenomenon was found: the formation of multilayered spots of thicker films in the wetting film of aqueous solutions of Silwet L-77 on hydrophobic surfaces. An expansion of the thicker spots within the film and the formation of "channels" between the spots and the edge of the film led to a continuous shrinkage of the wetting film and its disappearance in the end. We suggested that the multiple thicker films originate from the multilayer structuring of trisiloxane bilayers within the wetting film.

Wetting of low-energy surfaces.

Results of current theoretical methods for the calculation of contact angles on low-energy surfaces as functions of composition of solution and surface properties are reviewed.

Electrokinetic properties of methylated quartz capillaries.

Electrokinetic (zeta)-potentials of methylated (trimethylchlorosilane) quartz capillaries (5-6 microm in radius) were determined in 10(-4) M KCl solution. Over the course of time, the absolute values of the zeta-potential decrease, as a result of the formation of small bubbles on the rough methylated surface, generated from the flowing, nitrogen gas-saturated solution. This decrease is attributed to screening of a part of the solid surface. After the passage of time, a sharp increase in the zeta-potentials was observed, as the pressure was increased and the initial potential values were recovered. Sometimes, oscillations in the zeta-potentials were observed. This behaviour was explained by detachment of bubbles from the methylated surfaces by the flowing solution. Addition of non-ionic surfactant, which made the methylated surface hydrophilic, decreased the measured zeta-potentials. This was attributed to suppression of water slippage, an effect known to occur for hydrophobic solid surfaces. A mixed mechanism of charge formation is characteristic for these methylated quartz surfaces and is connected with presence of hydrophobic and hydrophilic areas. The ratio between these areas controls both the formation of surface charge as well as the contact angles.

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.

Electrosurface properties of poly(ethylene terephtalate) track membranes.

The electrosurface properties of track membranes made from poly(ethylene terephtalate) were determined by streaming potential (SP) and electron spin resonance (ESR) methods. The membrane pore diameters were in range from 20 to 200 nm. The pore electrical charge surface density values calculated with data obtained with methods mentioned differed dramatically. This difference was explained with the gel layer located at pore surface. The pore possesses a high large surface amassing a large electrical charge arising as a result of carboxylic groups dissociation. This charge cannot be detected with SP method due to high hydraulic resistance of the gel layer. In contrast to SP method, ESR detects all active centres parental of electrical charge at gel surface.

Disjoining pressure of thin nonfreezing interlayers.

At subzero temperatures, it has been found that nonfreezing water interlayers form between the inner smooth surface of a thin quartz capillary and the ice in the capillary core. The dilatometry method has been used to measure the thicknesses of these nonfreezing interlayers over a range of temperatures between -1 and -0.14 degrees C and for applied pressures up to 8 MPa. The measured data are used to calculate the disjoining pressure isotherms of nonfreezing water interlayers. It is shown that structural forces caused by structural changes of nonfreezing water are the main contribution to the disjoining pressure. The structural changes are also responsible for the phenomenon of the nonfreezing interlayers.

Spreading of liquid drops over saturated porous layers.

Spreading of small liquid drops over thin porous layers saturated with the same liquid is investigated from both theoretical and experimental points of view. A theory is presented that shows that spreading is governed by the same power law as in the case of spreading over a dry solid substrate. The Brinkman's equations are used to model the liquid flow inside the porous substrate. An equation of the drop spreading is deduced, which shows that both an effective lubrication and the liquid exchange between the drop and the porous substrates are equally important. The presence of these two phenomena removes the well-known singularity at the moving three-phase contact line. Matching of the drop profile in the vicinity of the three-phase contact line with the main spherical part of the drop gives the possibility to calculate the pre-exponential factor in the spreading law via permeability and effective viscosity of the liquid in the porous layer. Unfortunately, the latter dependency turns out to be very weak. Spreading of silicone oils over different microfiltration membranes is carried out. Radii of spreading on time experimental dependencies confirm the theory predictions. Experimentally found coefficients agree with theoretical estimations.

Spreading of aqueous SDS solutions over nitrocellulose membranes.

Experimental investigations were carried out on the spreading of small drops of aqueous SDS solutions over dry thin porous substrates (nitrocellulose membranes) in the case of partial wetting. The time evolution was monitored of the radii of both the drop base and the wetted area inside the porous substrate. The total duration of the spreading process was subdivided into three stages: the first stage: the drop base expands until the maximum value of the drop base is reached, the contact angle rapidly decreases during this stage; the second stage: the radius of the drop base remains constant and the contact angle decreases linearly with time; the third stage: the drop base shrinks and the contact angle remains constant. The wetted area inside the porous substrate expends during the whole spreading process. Appropriate scales were used with a plot of the dimensionless radii of the drop base, of the wetted area inside the porous substrate and the dynamic contact angle on the dimensionless time. Our experimental data show: the overall time of the spreading of drops of SDS solution 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 hydrodynamic reasons. It is shown using independent spreading experiments of the same drops on nonporous nitrocellulose substrate 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.

Spreading of liquid drops over dry porous layers: complete wetting case.

Spreading of small liquid drops over thin dry porous layers is investigated from both theoretical and experimental points of view. Drop motion over a porous layer is caused by an interplay of two processes: (a) the spreading of the drop over already saturated parts of the porous layer, which results in an expanding of the drop base; (b) the imbibition of the liquid from the drop into the porous substrate, which results in a shrinkage of the drop base and an expanding of the wetted region inside the porous layer. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations is derived to describe the evolution with time of 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 are used for an independent determination of these two parameters. The system of differential equations does not include any fitting parameter after these two parameters are determined. Experiments were carried out on the spreading of silicone oil drops over various dry 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 are monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to our theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) 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 conclusions again are in good agreement with our experimental observations.