Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

Surfactant Control of Coffee Ring Formation in Carbon Nanotube Suspensions

The coffee ring effect regularly occurs during the evaporation of colloidal droplets and is often undesirable. Here we show that adding a specific concentration of a surfactant can mitigate this effect. We have conducted experiments on aqueous suspensions of carbon nanotubes that were prepared with cationic surfactant dodecyltrimethylammonium bromide added at 0.2, 0.5, 1, 2, 5, and 10 times the critical micelle concentration. Colloidal droplets were deposited on candidate substrates for printed electronics with varying wetting characteristics: glass, polyethylene terephthalate, fluoroethylene propylene copolymer, and polydimethylsiloxane. Following drying, four pattern types were observed in the final deposits: dot-like, uniform, coffee ring deposits, and combined patterns (coffee ring with a dot-like central deposit). Evaporation occurred predominantly in constant contact radius mode for most pattern types, except for some cases that led to uniform deposits in which early stage receding of the contact line occurred. Image analysis and profilometry yielded deposit thicknesses, allowing us to identify a coffee ring subfeature in all uniform deposits and to infer the percentage coverage in all cases. Importantly, a critical surfactant concentration was identified for the generation of highly uniform deposits across all substrates. This concentration resulted in visually uniform deposits consisting of a coffee ring subfeature with a densely packed center, generated from two distinct evaporative phases.

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures

The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of -10 to -30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid-liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

The core composition of DNA block copolymer micelles dictates DNA hybridization properties, nuclease stabilities, and cellular uptake efficiencies

Here, we report how the nature of the hydrophobic core affects the molecular interactions of DNA block copolymer assemblies. Three different amphiphilic DNA block copolymers, DNA-b-polystyrene (DNA-b-PS), DNA-b-poly(2-vinylpyridine) (DNA-b-P2VP), and DNA-b-poly(methyl acrylate) (DNA-b-PMA) were synthesized and assembled into spherical micelles composed of a hydrophobic polymer core and DNA corona. Interestingly, DNA block copolymer micelles having different hydrophobic cores exhibited markedly different molecular and biological interactions. DNA-b-PS exhibited higher melting temperature, sharper melting transition, higher stability to nuclease-catalyzed DNA degradation, and higher cellular uptake efficiency compared to DNA-b-P2VP and DNA-b-PMA. The investigation of the self-assembly behavior revealed a much higher aggregation number and DNA density for DNA-b-PS micelles, which explains the superior properties of DNA-b-PS. These results demonstrate that the type of the hydrophobic core polymer, which has been largely overlooked, has a profound impact on the molecular and biological interactions of the DNA shell.

Correlations between the Type of Aggregates in the Bulk Phase and the Functionality and Safety of All-Purpose Cleaners

The article shows that the type and concentration of inorganic salt can be translated into the structure of the bulk phase and the performance properties of ecological all-purpose cleaners (APC). A base APC formulation was developed. Thereafter, two types of salt (sodium chloride and magnesium chloride) were added at various concentrations to obtain different structures in the bulk phase. The salt addition resulted in the formation of spherical micelles and-upon addition of more electrolyte-of aggregates having a lamellar structure. The formulations had constant viscosities (ab. 500 mPa·s), comparable to those of commercial products. Essential physical-chemical and performance properties of the four formulations varying in salt types and concentrations were evaluated. It was found that the addition of magnesium salt resulted in more favorable characteristics due to the surface activity of the formulations, which translated into adequately high wettability of the investigated hydrophobic surfaces, and their ability to emulsify fat. A decreasing relationship was observed in foaming properties: higher salt concentrations lead to worse foaming properties and foam stability of the solutions. For the magnesium chloride composition, the effect was significantly more pronounced, as compared to the sodium chloride-based formulations. As far as safety of use is concerned, the formulations in which magnesium salt was used caused a much lesser irritation compared with the other investigated formulations. The zein value was observed to decrease with increasing concentrations of the given type of salt in the composition.

Synthesis and Properties of a Quaternary Ammonium Salt Gemini Surfactant with Diethyl Ether as the Spacer Group

A quaternary ammonium salt gemini surfactant (C12N)2O with diethyl ether as the spacer group was synthesized by dodecyl dimethyl amine and 2-chloroethyl ether. The structure of the product was confirmed by Fourier transform infrared spectra (FT-IR) and proton nuclear magnetic resonance spectra (1H NMR), which showed that the structure of the synthesized product was consistent with the theoretical structure. We tested the surface tension of (C12N)2O at 25°C and found the critical micelle concentration (CMC) was roughly 2 orders of magnitude lower than that of dodecyl trimethylamine chloride (DTAC). Gemini structure and the unique diethyl ether spacer group endowed the novel amphiphile excellent properties including low contact angle and high antistatic property. (C1 2N)2O showed good compatibility with alcohol ether sulfate (AES) and dodecylbenzene sulfonate (LAS) in surfactant mixed systems. When the ratio of (C12N)2O:AES was 1:4 and (C12N)2O:LAS 1:9, the emulsifying properties were preferable to single surfactant.

Reconfigurable Microcube Assemblies at the Liquid/Air Interface: The Impact of Surface Tension on Orientation and Capillary-Force-Interaction-Driven Assembly

The systematic investigation of the dependence of the orientation and capillary interaction of hydrophobized polystyrene microcubes at the liquid/air interface on the surface tension of the aqueous subphase is reported. By decreasing the subphase surface tension, the preferential orientation of the cubes was observed to change independent of the surfactant type from the vertex up to the edge up and finally to the face up. Concomitantly, the structure of the aggregates obtained by cube assembly was observed to change from a close-packed hexagonal to tilted linear and finally to flat plate. In particular, the preferential orientation of the cubes was virtually independent of the surfactant charge at a constant surface tension. In addition, reconfigurable microcube assemblies at the liquid/air interface, which respond to the surface tension of the subphase, were observed for the first time. The dynamic reconfigurability of preformed microcube aggregates induced by adding surfactant to the subphase may open new pathways to dynamic assemblies at liquid/air interfaces, which may be interesting, e.g., for sensing applications.

A numerical model of superspreading surfactants on hydrophobic surface

Many contributions significantly on experimental and mathematical studies are made to understand the mechanism of superspreading. Only few numerical methods have been proposed which solve the system of equations with soluble and insoluble surfactants. Among them, we propose a computational fluid dynamics model, based on the volume of fluid technique, with the piecewise linear interface calculation method. Interface reconstruction is applied to simulate the time evolution of the dynamics of drop spreading of surfactants on a thin water layer. We have allowed the occurrence of both the regimes relating to a series of trisiloxane (M(D′EnOH)M), sodium dodecyl sulphate, and Tergitol NP10 surfactants drop on a thin water layer with the influence of Marangoni stress. The numerical data seem consistent with those experimental for both regimes. It validates predictions for the spreading exponent in which the law of the radius of the circular area covered by the surfactant grows as tα, where 0 < α < 1. The comparison of the numerical and experimental predictions by Lee et al. [“Spreading of trisiloxanes over thin aqueous layers,” Colloid J. 71, 365–369 (2009)] is well represented in both regimes. The numerical study confirms that the spreading rates during the first stage increase as the solubility increases. This finding suggests that the model is adequate for describing the spreading of surfactants on thin fluid layers.

Self-Assembled Curved Macroporous Photonic Crystal-Based Surfactant Detectors

Surfactants are extensively used as detergents, dispersants, and emulsifiers. Thus, wastewater containing high-concentration surfactants discharged to the environment pose a serious threat to the ecosystem. Unfortunately, conventional detection methods for surfactants suffer from the use of sophisticated instruments and cannot perform detections for various surfactants by a single analysis. The article reports the development of simple and sensitive surfactant detection using doctor-blade-coated three-dimensional curved macroporous photonic crystals on a cylindrical rod. The photonic crystals exhibit different hydrophobicities at various angular positions after surface modification. The penetration of aqueous surfactant solutions in the interconnected macropores causes red-shift as well as reduction in amplitude in the optical stop bands, resulting in surfactant detection with visible readout. The correlation between the surface tension, as well as the solution-infiltrated angular position, and the concentration of aqueous surfactant solutions has also been investigated in this study.

Influence of surfactants in forced dynamic dewetting

In this work we show that the forced dynamic dewetting of surfactant solutions depends sensitively on the surfactant concentration. To measure this effect, a hydrophobic rotating cylinder was horizontally half immersed in aqueous surfactant solutions. Dynamic contact angles were measured optically by extrapolating the contour of the meniscus to the contact line. Anionic (sodium 1-decanesulfonate, S-1DeS), cationic (cetyl trimethylammonium bromide, CTAB) and nonionic surfactants (C₄E₁, C₈E₃ and C₁₂E₅) with critical micelle concentrations (CMCs) spanning four orders of magnitude were used. The receding contact angle in water decreased with increasing velocity. This decrease was strongly enhanced when adding surfactant, even at surfactant concentrations of 10% of the critical micelle concentration. Plots of the receding contact angle-versus-velocity almost superimpose when being plotted at the same relative concentration (concentration/CMC). Thus the rescaled concentration is the dominating property for dynamic dewetting. The charge of the surfactants did not play a role, thus excluding electrostatic effects. The change in contact angle can be interpreted by local surface tension gradients, i.e. Marangoni stresses, close to the three-phase contact line. The decrease of dynamic contact angles with velocity follows two regimes. Despite the existence of Marangoni stresses close to the contact line, for a dewetting velocity above 1-10 mm s^-1 the hydrodynamic theory is able to describe the experimental results for all surfactant concentrations. At slower velocities an additional steep decrease of the contact angle with velocity was observed. Particle tracking velocimetry showed that the flow profiles do not differ with and without surfactant on a scales >100 μm.

Control of stain geometry by drop evaporation of surfactant containing dispersions

Control of stain geometry by drop evaporation of surfactant containing dispersions is an important topic of interest because it plays a crucial role in many applications such as forming templates on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials. This paper presents a review of the published articles on the diffusive drop evaporation of pure liquids (water), the surfactant stains obtained from evaporating drops that do not contain dispersed particles and deposits obtained from drops containing polymer colloids and carbon based particles such as carbon nanotubes, graphite and fullerenes. Experimental results of specific systems and modeling attempts are discussed. This review also has some special subtopics such as suppression of coffee-rings by surfactant addition and "stick-slip" behavior of evaporating nanosuspension drops. In general, the drop evaporation process of a surfactant/particle/substrate system is very complex since dissolved surfactants adsorb on both the insoluble organic/inorganic micro/nanoparticles in the drop, on the air/solution interface and on the substrate surface in different extends. Meanwhile, surfactant adsorbed particles interact with the substrate giving a specific contact angle, and free surfactants create a solutal Marangoni flow in the drop which controls the location of the particle deposition together with the rate of evaporation. In some cases, the presence of a surfactant monolayer at the air/solution interface alters the rate of evaporation. At present, the magnitude of each effect cannot be predicted adequately in advance and consequently they should be carefully studied for any system in order to control the shape and size of the final deposit.

Contact angles of surfactant solutions on heterogeneous surfaces

Exploration of new model for contact angle of surfactant solutions on smooth/rough heterogeneous surfaces, allowing adsorption at all interfaces.

Dynamic wetting of hydrophobic polymers by aqueous surfactant and superspreader solutions

In this paper, we comparatively investigated the wetting performance of aqueous surfactant solutions in a wide range of concentrations, including conventional ionic surfactants (CTAB, SDS) and two nonionic polyether-modified trisiloxane surfactants (TSS6/3, TSS10/2), over hydrophobic polypropylene substrates. In all cases, scaling analysis of the experimental data of spreading drops showed that the early spreading stage was dominated by inertia and that the duration of this stage was not influenced by the addition of surfactant. For conventional surfactant solutions, we only observed the inertia-dominated spreading stage before the drops stopped wetting with a finite stable contact angle. For both trisiloxane surfactants, after the inertial stage we observed a second viscosity-dominated spreading stage. In this stage, TSS10/2 showed an enhanced wetting capability independent of its concentration, while TSS6/3 started to show a concentration-dependent spreading behavior that was fully developed in a third superspreading stage. Our findings suggest that the superspreading property of TSS6/3 began to take effect after a characteristic time, before which the superspreading TSS6/3 and the nonsuperspreading TSS10/2 behaved similarly. Power law fits to the superspreading regime are in agreement with an interpretation of Marangoni flows resulting from surface tension gradients.

New Detergency Aspects through Visualisation of Soil Release Polymer Films on Textile Surfaces

Soiling degree and cleanability of different polyester textile materials before and after impregnation with a soil release polymer (SRP) has been systematically investigated. The use of an imaging instrument for the optical roughness analysis operating on the principle of chromatic aberration allowed visualisation of a transparent SRP film on impregnated textile surfaces. Through visualisation of the SRP film and quantification of its relative area, a possible mechanism of cleanability due to impregnation with SRP is proposed.

Soiling Degree and Cleanability of Differently Treated Polyester Textile Materials

Soiling degree and cleanability of well-characterized polyester textile materials treated with different soil release polymers (SRP) have been systematically investigated. It was shown that soil removal considered in terms of soiling additional density (SAD) strongly depends on the kind of textile materials investigated. The influence of textiles surface properties such as the degree of hydrophobicity and topographical structure on spreading of water before and after the treatment with SRP was quantified, as well.

Cleanability of Plastic Flooring Materials Related to their Surface Properties

The purpose of this study was to test the feasibility of contact angle measurement in evaluating soilability and cleanability of 11 different surfaces of flooring materials. The surface properties were examined using an expanding contact angle technique and the cleanability was evaluated colorimetrically. The values of advancing contact angles θA were related to the soil residues on particle-soiled surfaces and oil-soiled surfaces. The results indicate the potential of contact angle measurements to evaluate cleanability of flooring materials.

Influence of the Cross-sectional Geometry on Wettability and Cleanability of Polyester Woven Fabrics

Filament cross-sections used in textiles and composites are becoming more and more complex. The type of cross-sectional shape impacts filament properties and are therefore, yarn and fabric characteristics. In this paper, the influence of different filament cross-section geometry on fibre properties as well as on fabric surface characteristics was studied. PET (polyethylene terephthalate) filament yarns made from two different cross-sectional shaped filaments, round and cruciform, were manufactured by the melt spinning process. Polyester fabrics were manufactured in a needle band weave machine from these two types of filament yarns. Topographic characteristics of polyester fabrics manufactured were determined by an imaging instrument for optical roughness analysis based on the principle of chromatic aberration. Differences between soiling behaviour, cleanability and wettability of the fabrics were revealed and discussed.

Influence of surfactant concentration and background salt on forced dynamic wetting and dewetting

Forced wetting and dewetting of polymer surfaces in aqueous solutions containing cationic surfactant cetyltrimethylammonium bromide (CTAB) has been studied with a rotating cylinder half immersed in the solution. The receding contact angle decreases with faster withdrawing speeds. This decrease is enhanced when adding CTAB. The addition of salt to the CTAB solution further enhances the effect but does not have a significant effect alone. We interpret this change in the dynamic contact angle with a surfactant-induced Marangoni effect.

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.

Autophilic effect: wetting of hydrophobic surfaces by surfactant solutions

This paper resolves questions in the literature regarding the autophilic effect (i.e., movement of surfactant past the advancing contact line-leading to an increase in drop radius beyond that due to the advance) and its importance to quasi-static sessile drop wetting. Various systems (SDS, HTAB, and MEGA 10 surfactant solutions at three concentrations each and pure water and ethylene glycol on hydrophobic Teflon and OTS-coated silicon) are probed to determine the existence, time constant, and magnitude of the autophilic effect, using quasi-static advancing and receding sessile drops. From spreading results and advancing contact angle measurements, it is inferred that the autophilic effect does not occur for our systems (in contradiction of some literature) for the following reasons. First, no relation exists between the time constant for spreading and surfactant concentration, meaning the spreading seen is likely inertial in cause and not due to surfactants. Second, advancing contact angle decreases between tests on clean surfaces and those pre-exposed to surfactant, ruling out the possibility that the autophilic effect is faster than the advance. Third, spreading is seen after the end of the advance over both clean and pre-exposed surfaces, ruling out the possibility that the autophilic effect is slower than the advance. Finally, the pure liquids spread in a similar fashion to surfactant solutions on Teflon and similar contact angle measurements are seen for surfactant solutions and pure liquids of similar surface tension.

Impact of shear-thinning and yield-stress drops on solid substrates

The behaviour of shear-thinning and viscoplastic fluid drops impacting on solid substrates as compared with that of Newtonian drops is studied experimentally by means of high-speed imaging. In particular, the investigation focuses on the morphological aspects of drops after inertial spreading. While the impact morphology of drops of shear-thinning fluids turns out to be qualitatively similar to that of Newtonian fluids, viscoplastic drops can exhibit central drop peaks at the end of inertial spreading. The influence of yield-stress magnitude on drop impact behaviour is qualitatively established by measuring the size of these central drop peaks. The peaks indicate that drop deformation during impact is localized: within a threshold radius, shear-stress effects will not be large enough in magnitude to overcome yield-stress effects, and therefore viscoplastic fluids within this region will not deform from the drop shape prior to impact.

Parametric verification of one-way lithographic wicks

An asymmetric gap was fabricated into a dense array of medium-high carbon nanotubes to achieve a one-way fluid transport device through capillary action.

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.

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.