V. D. Sobolev

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.

Prediction of contact angles on the basis of the Frumkin-Derjaguin approach

Abstract The Frumkin-Derjaguin approach was used for the calculation of contact angles on the basis of isotherms of disjoining pressure of wetting films. The contact angles are calculated for a queous solutions of electrolytes of low concentration on quartz surface, taking into account forces of molecular repulsion, electrostatic attraction and structural forces. A high degree of surface hydrophilicity may be attained under the predominant action of repulsive structural forces. Large contact angles are caused by the predominant action of hydrophobic attraction forces and/or electrostatic attraction forces acting between two oppositely charged film interfaces. At a moderate degree of hydrophilicity, in the range of contact angles from 10–15° to 50–60°, it is sufficient to take into account the molecular and electrostatic forces only. This determines the region of applicability of the DLVO theory. The parameters of the short-range hydrophobic forces seem to be the same in wetting films and in water interlayers between two hydrophobic surfaces.

Examination of the surface of quartz capillaries by electrokinetic methods

Abstract The electrokinetic and surface charge of “fresh” quartz capillaries is equal to 1.2 ÷ 2.3 μC/cm2, and it is independent of the concentration of KCl solution (for C ⩾ 103 M). Long-time contact with water and KCl solutions leads to the gel-layer formation on the capillary surface, and accordingly to the changes in surface charge and potential. The liquid flow in the capillary under the action of a sufficiently high pressure drop can shear and tear off the surface gel layer. This effect is accompanied by the observed changes in the ζ-potential values.

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.

Influence of cationic surfactant on the surface charge of silica and on the stability of aqueous wetting films

Abstract Interference and ellipsometric methods were used to measure the equilibrium thicknesses of wetting films (50–70 nm) of aqueous solutions of CTAB and NaCI. The experimental isotherms of disjoining pressure of wetting films agree with the results of calculations performed according to the theory of electrostatic forces acting between film surfaces whose potentials differ in value. The ranges of the stability, metastability, and instability of the films were determined. The rupturing of metastable films is accompanied by a transition from complete to partial wetting.

Viscosity of nonfreezing thin interlayers between the surfaces of ice and quartz

Abstract From the shift rate of the ice column in thin quartz cylindrical capillaries the value of h η has been measured at different temperatures below 0°C. The viscosity values of nonfreezing water and KCI solution interlayer η are calculated on the basis of the known values of the interlayer thickness h. For the electrolyte solution, the h values are much higher than those for pure water.

On the non-freezing water interlayers between ice and a silica surface

Abstract A capillary dilatometer was used for measuring the thickness H of non-freezing water interlayers between ice and the surfaces of silica sol particles (radius R = 68, 56 and 16 nm), and between ice and the surfaces of very thin quartz capillaries (radius r ≈ 1 μm). Reversible H ( T ) dependences are obtained in the temperature range from −0.2 to – 1.5°C. The values of H ( T ) increase with increasing radius of the sol particles and with pressure P . Values of the heat of ice melting calculated from experimental data using the Clapeyron-Clausius equation are close to the known values for bulk ice. Values of the viscosity of non-freezing water interlayers were assessed on the basis of the measured shift rates of ice columns in quartz capillaries. In the region where t > −0.5°C and H > 10 nm the viscosity is about 2–3 times higher compared with that of bulk water.

Zeta potential measurements with fibre plugs in 1:1 electrolyte solutions

Abstract The aim of this paper is correct calculation of zeta potential values from streaming potential measurements in aqueous electrolyte solutions. To determine correct zeta potential values it is very important to measure the total electric conductivity exactly and to consider the electrode polarization. The streaming potential measurements of various fibre samples in KCl solution of different concentrations were carried out by means of two different measuring devices working in very different pressure ranges. The calculated zeta potential values show that the measuring method gives well reproducible results independent of the applied pressure and the kind of device. In addition, we discuss the possibility of, and the reason for, a maximum observed sometimes in the dependence of zeta potential values on the electrolyte concentration in the case of 1 : 1 electrolytes by means of measurements of a fibre diaphragm and by using computer simulations based on a Gouy—Chapman—Stern—Grahame model.

Modification of quartz surfaces using cationic surfactant solutions

Abstract Two models of Langmuir-type adsorption of cationic surfactant on quartz surfaces were developed. The monolayer model considers a simultaneous absorption of cations on charged and neutral sites. The two-layer model assumes that absorption takes place on charged sites only, and a second layer is formed as a result of adsorption of cations on hydrophobic tails of preadsorded ions. Computer treatment of experimental dependencies of the surface charge of quartz capillaries on the concentration of cethyltrimethylammonium bromide (CTAB) solutions and pH values results in potentials of adsorption on charged sites Φ 1 =−15 kT, and on non-charged ones Φ 2 =−11 kT. Both models give the same values of potentials, probably due to close values of Φ 2 for non-charged sites on quartz surfaces and for hydrophobic tails for preadsorbed cations. Atomic force microscopy of adsorded CTAB layers favors the monolayer mechanism of adsorption, no patches of molecules were observed.

Electrokinetic study of polymer surfaces

Abstract On the basis of electrokinetic measurements in thin quartz capillaries coated with micrometer-thick polystyrene (PS) layers, values of ζ-potentials and of surface charge σ in different electrolyte solutions were calculated. Potentials of adsorption of K+, Ba2+, La3+, and Cl- ions on a hydrophobic PS surface were computed with equations of localized adsorption. For hydrophilic copolymers of PS with polyacrylic acid (PA) the mechanism of charge formation is connected with dissociation of COOH groups. For 4% of PA in copolymer, the surface charge increases to 4 μC/cm2 at pH 11, which is 10-fold as compared with PS. A further increase in PA content (up to 40%) causes the formation of charged surface “brush” layers. Dissociation of COOH groups within a brush layer leads to very high values of surface charge (up to −160 μC/cm2) and surface conductivity. The relative value of surface conductivity decreases from κ s κ v = 3.2 to 1 when the concentration of KC1 solution increases from 10−4 to 10−1N.