Natalia A. Ivanova

Kinetics of wetting and spreading by aqueous surfactant solutions.

Interest in wetting dynamics processes has immensely increased during the past 10-15 years. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant, a surface-active polymer, a polyelectrolyte or their mixture are added to a liquid (usually water). The main idea of the paper is to give an overview on some dynamic wetting and spreading phenomena in the presence of surfactants in the case of smooth or porous substrates, which can be either moderately or highly hydrophobic surfaces based on the literature data and the authors own investigations. Instability problems associated with spreading over dry or pre-wetted hydrophilic surfaces as well as over thin aqueous layers are briefly discussed. Toward a better understanding of the superspreading phenomenon, unusual wetting properties of trisiloxanes on hydrophobic surfaces are also discussed.

Fluoro- vs hydrocarbon surfactants: why do they differ in wetting performance?

Fluorosurfactants are the most effective compounds to lower the surface tension of aqueous solutions, but their wetting properties as related to low energy hydrocarbon solids are inferior to hydrocarbon trisiloxane surfactants, although the latter demonstrate higher surface tension in aqueous solutions. To explain this inconsistency available data on the adsorption of fluorosurfactants on liquid/vapour, solid/liquid and solid/vapour interfaces are discussed in comparison to those of hydrocarbon surfactants. The low free energy of adsorption of fluorosurfactants on hydrocarbon solid/water interface should be of a substantial importance for their wetting properties.

Spreading of Aqueous Solutions of Trisiloxanes and Conventional Surfactants over PTFE AF Coated Silicone Wafers

Kinetics of spreading of aqueous trisiloxane surfactant T(n) (with n = 4, 6, and 8 ethoxy groups) solutions and conventional aqueous surfactant solutions (Tween 20, C12E4, SDS) over silicon wafers coated with PTFE AF is experimentally investigated. It has been found that trisiloxane solutions spread on highly hydrophobic PTFE AF coated silicone wafers; however, they do not show superspreading behavior on these highly hydrophobic substrates. Solutions of conventional nonionic surfactants investigated show kinetics of spreading similar to trisiloxanes. Three regimes of spreading have been identified (i) complete non-wetting during the spreading process at low concentrations, (ii) a transition from initial nonwetting to partial wetting at the end of the spreading process at intermediate concentrations, and (iii) partial wetting both at the beginning and the end of the spreading process at higher concentrations. Transition from the first regime (i) to the second regime (ii) takes place at the critical aggregation concentration (CAC) or critical micelle concentration (CMC), transition from regime (ii) to regime (iii) happens at the critical wetting concentration (CWC). In the case of regime (i) the spreading of nonionic surfactants solutions investigated on PTFE AF coated silicone wafers is slow and follows a theoretically predicted law (Starov; et al. J. Colloid Interface Sci. 2000, 227 (1), 185). In the case of regimes (ii) and (iii), the spreading of the nonionic surfactant solutions investigated proceeds in two stages: the fast short first stage, which is followed by a much slower second stage. It is shown that the slow stage develops according to a previously described theoretical model. According to this theory the surfactant molecules adsorb in front of the moving three-phase contact line (autophilic phenomenon), which results in a partial hydrophilisation of an initially hydrophobic substrate and a spreading as a consequence. We assume that the first stage of the spreading is related to the disintegration of surfactant aggregates in the vicinity of the moving three-phase contact line.

Why do aqueous surfactant solutions spread over hydrophobic substrates

Spreading of aqueous surfactant solution droplets over hydrophobic substrates proceeds in one slow stage at concentration of surfactants below some critical value and in two stages if the surfactant concentration is above the critical value: the fast and relatively short first stage is followed by a slower second stage. It is shown that the kinetics of a slow spreading at concentrations below the critical value and the second stage at concentrations above the critical value are determined by a transfer of surfactant molecules on a bare hydrophobic substrate in front of the moving three-phase contact line (autophilic phenomenon). The latter process results in an increase of the solid-vapour interfacial tension of the hydrophobic solid surface in front of the moving three-phase contact line and spreading as a result. It is proven that the adsorption of surfactant molecules in front of the moving three-phase contact line results in a decrease of the total free energy of the droplet. Hence, the adsorption of surfactants molecules on a bare hydrophobic substrate in front of the moving three-phase contact line is a spontaneous process in spite of an increase of the local solid-vapour interfacial tension. The duration of the first stage of spreading in the case of the surfactant concentration above the critical value correlates well with the duration of adsorption of surfactant molecules onto a liquid-vapour interface. The latter allows assuming that the adsorption on the liquid-vapour interface is the driving mechanism of spreading during the first fast stage of spreading at surfactant concentrations above the critical value. It is discussed why the first stage of spreading does not take place in the case of surfactant concentrations below the critical concentration in spite of the longer duration of adsorption on liquid-vapour interface in this case.

Spreading behaviour of aqueous trisiloxane solutions over hydrophobic polymer substrates

The kinetics of spreading of aqueous trisiloxane solutions over different solid hydrophobic substrates has been investigated experimentally. Two pure trisiloxane surfactants with 6 and 8 oxyethylene groups at concentrations close to the critical aggregation concentration and the critical wetting concentration were used in the spreading experiments. Three hydrophobic substrates (Teflon AF, Parafilm, and polystyrene) having different surface properties were used. It was found that the spreading behaviour depends on the hydrophobic/roughness properties of substrates. The rapid spreading and complete wetting were observed for both trisiloxane surfactant solutions at the critical wetting concentration on a substrate with a moderate hydrophobicity. For both highly hydrophobic Teflon AF and Parafilm substrates only partial wetting was found. The experiments have shown that the spreading behaviour over all substrates proceeds at two stages. At the critical aggregation concentration for both trisiloxanes on all substrates the time lag of the spreading was detected.

Removal of micrometer size particles from surfaces using laser-induced thermocapillary flow: Experimental results.

HYPOTHESIS Reducing particle contaminations on solid and delicate surfaces is of great importance in a number of industries. A new non-destructive method is proposed, which is based on the laser-induced thermocapillary effect for the removal of micron size particles from surfaces. The cleaning mechanism is related to the surface-tension-driven flows produced by the laser heating of thin layer of a cleaning liquid deposited onto a surface contaminated with particles. EXPERIMENTS Focusing the laser irradiation into the line laser beam allowed using this method for a large-scale cleaning of surfaces. Hexadecane was used as a cleaning liquid to remove micron-sized polyethylene, Teflon, talc and Al2O3 particles from surfaces of welding glass, carbolite and soft magnetic disc using the line beam of the IR laser. FINDINGS A good cleaning efficiency was achieved for cases of polyethylene and Teflon particles on both the complete wettable welding glass and the low-wettable soft magnetic disc, while in case of oleophilic talc and Al2O3 particles the effectiveness of the cleaning method was lower on all three substrates investigated. The thermal influence of the laser irradiation on substrates used was measured with infrared camera. It was shown that temperature in the irradiated area during the long-time heating increases insignificantly and cannot cause any damage of the substrate.

Varifocal liquid lens actuated by laser-induced thermal Marangoni forces

This paper proposes a simple and easily implemented configuration of a varifocal liquid lens actuated by laser-induced thermal Marangoni forces. The lens consists of a sessile droplet of low-volatile liquid placed on a transparent solid surface. The focal length tunability is achieved by changing a local curvature of the droplet surface due to the thermocapillary displacement of liquid from the core part of the droplet to its edge caused by heating with a laser beam. Depending on the power of the laser beam, the droplet can operate as a varifocal convergent or varifocal divergent lens. In the convergent mode, the curvature of the droplet surface varies from an initial positive value to a near zero value (flat surface), whereas in the divergent mode, the curvature changes from a positive value to a negative value due to the formation of a thermocapillary dimple in the droplet.

FTIR ATR Study of Adsorption of Trisiloxanes and Hydrocarbon Surfactants at Hydrophobic Solids from Aqueous Solutions

Kinetics of the integrated absorbance in C−H stretch region for ethoxylated trisiloxane and alkyl polyethoxylate surfactants on low- and highly hydrophobic surfaces has been measured by Fourier-Transform Infrared spectroscopy (FTIR) in the attenuated total reflection (ATR) mode. It has been found that regardless of the surface energy of substrate the absorbance of trisiloxanes continuously increases during the experiment (characteristic time scale is ten minutes), while the absorbance of alkyl polyethoxylate surfactants reaches equilibrium for tens of seconds on low hydrophobic and for a few minutes on the highly hydrophobic surfaces. The continuous growth of absorbance with increasing bulk concentration of surfactants has been detected in the case of trisiloxanes on both substrates even at concentrations above critical wetting concentration; while hydrocarbon surfactants attained the constant values of absorbance at concentrations above critical aggregation concentration. The results for alkyl polyethoxylate surfactants obtained from FTIR-ATR spectra are consistent with the results obtained by other authors used neutron and optical reflectometry. Influence of the length of hydrophilic chains on the value of the absorbance of surfactants has been analyzed. In the case of alkyl polyethoxylate surfactants the absorbance decreases with increasing the length of hydrophilic ethoxy groups. However, in the case of trisiloxane surfactants studied the above trend cannot be clearly traced. Some aspects associated with the relationship between adsorption of trisiloxanes at hydrophobic solid/liquid interfaces and spreading kinetics are discussed.