Droplets Of Surfactant Solutions Research Articles

Solutal Marangoni augmented levitation time of an aqueous surfactant drop over an immiscible oil pool

Droplets are found to exhibit a finite residence time over the volatile liquid pool due to the confined evaporation mediated sustained thin film draining. The levitated drops are quite critical in biomedical applications such as for the biophysical micro reactors. It is hypothesized that the presence of surfactants can induce solutal Marangoni and can further enhance the levitation time. Moreover, the surfactant drops can closely mimic the bio drop assay. Droplets of surfactant solutions are allowed to impact over a series of volatile hydrocarbon oils to understand the effect of surfactant concentration, impact height and the properties of the oil pool using high speed imaging technique. Flow visualization is employed in the pendant droplet mode to understand the Marangoni induced internal circulation. The droplets of surfactant solutions are found to exhibit enhanced residence time compared to the case of non-surfactant drops. The levitation time is a function of the surfactant concentration and the volatility of the oil pool. A mathematical model is developed incorporating the effect of solutal Marangoni to test the hypothesis and is found to predict the levitation time with reasonable accuracy. A universal floating–sinking regime map was developed incorporating the influence of governing parameters.

Surfactants Improving the Wetting Behavior and Adhesion Mechanism of Pesticide Dilution Droplets on Jujube Leaf Surfaces.

Fruit tree leaves have different chemical compositions and diverse wax layer structures that result in different patterns of wetting and pesticide solution spreading on their surface. Fruit development is a time when pests and diseases occur, during which a large number of pesticides are needed. The wetting and diffusion properties of pesticide droplets on fruit tree leaves were relatively poor. To solve this problem, the wetting characteristics of leaf surfaces with different surfactants were studied. The contact angle, surface tension, adhesive tension, adhesion work, and solid-liquid interfacial tension of five surfactant solution droplets on jujube leaf surfaces during fruit growth were studied by the sessile drop method. C12E5 and Triton X-100 have the best wetting effects. Two surfactants were added to a 3% beta-cyfluthrin emulsion in water, and field efficacy tests were carried out on peach fruit moths in a jujube orchard at different dilutions. The control effect is as high as 90%. During the initial stage when the concentration is low, due to the surface roughness of the leaves, the surfactant molecules adsorbed at the gas-liquid and solid-liquid interfaces reach an equilibrium, and the contact angle on the leaf surface changes slightly. With increasing surfactant concentration, the pinning effect in the spatial structure on the leaf surface is overcome by liquid droplets, thereby significantly decreasing the contact angle. When the concentration is further increased, the surfactant molecules form a saturated adsorption layer on the leaf surface. Due to the existence of a precursor water film in the droplets, surfactant molecules on the interface continuously move to the water film on the surface of jujube tree leaves, thus causing interactions between the droplets and the leaves. The conclusion of this study provides theoretical guidance for the wettability and adhesion of pesticides on jujube leaves, so as to achieve the purpose of reducing pesticide use and improving pesticide efficacy.

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.

Rapid Spreading of a Droplet on a Thin Soap Film.

We study the spreading of a droplet of surfactant solution on a thin suspended soap film as a function of dynamic surface tension and volume of the droplet. Radial growth of the leading edge (R) shows power-law dependence on time with exponents ranging roughly from 0.1 to 1 for different surface tension differences (Δσ) between the film and the droplet. When the surface tension of the droplet is lower than the surface tension of the film (Δσ > 0), we observe rapid spreading of the droplet with R ≈ tα, where α (0.4 < α < 1) is highly dependent on Δσ. Balance arguments assuming the spreading process is driven by Marangoni stresses versus inertial stresses yield α = 2/3. When the surface tension difference does not favor spreading (Δσ < 0), spreading still occurs but is slow with 0.1 < α < 0.2. This phenomenon could be used for stretching droplets in 2D and modifying thin suspended films.

Drying Droplets with Soluble Surfactants.

We propose a theory for the drying of liquid droplets of surfactant solutions. We show that the added surfactant hinders droplet receding and facilitates droplet spreading, causing a complex behavior of the contact line of an evaporating droplet: the contact line first recedes, then advances, and finally recedes again. We also show that the surfactant can change the deposition pattern from mountain-like to volcano-like and then to coffee-ring-like. Specially, when the contact line motion undergoes a clear receding-advancing transition, a two-ring pattern is formed. The mechanism of the two-ring formation is different from the stick-slip mechanism proposed previously and may be tested experimentally.

Wetting Transition at a Threshold Surfactant Concentration of Evaporating Sessile Droplets on a Patterned Surface.

Wetting transitions induced by varying the components in a solution of a drying droplet can lead to its evolving shape on a textured surface. It can provide new insights on liquid pattern control through manipulating droplet solutions. We show the pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface. At low initial surfactant concentrations, the droplet maintains an octagonal shape until the end of drying. At intermediate initial surfactant concentrations, the early octagon spreads to a square, which later evolves to a stretched rectangle. At high initial surfactant concentrations, the droplet mainly exhibits the "octagon-to-square" transition, and the square shape is maintained until the end. The octagon-to-square transition occurs at similar temporal volume-averaged surfactant concentrations for the various initial surfactant concentrations. It results from the dependence of the surface energy change of spread over the micropyramid structure on the temporal volume-averaged surfactant concentration. At high initial surfactant concentrations, the accumulation of the surfactant near the contact line driven by outward flows could raise the local viscosity and enhance the pinning effect, leading to the great suppression of the "square-to-rectangle" transition.

Wetting and Spreading of Commercially Available Aqueous Surfactants on Porous Materials

The wetting properties of aqueous solutions of a commercially available surfactant at various concentrations on porous media are investigated using the KRUSS DSA100 shape analyzer and the ADVANCED software to process the data. Time evolution of both the contact angle and drop base diameter at each surfactant concentration after deposition were monitored. Three different porous substrates (sponges) were examined. The sponges used were a car sponge, dish sponge and audio sponge. The sponges were investigated both dry and at different degrees of saturation, that is, the amount of water absorbed into the sponge. It was found that pure distilled water droplets deposited on the dry porous media showed non-wetting. However, if droplets of surfactant solutions were deposited, then a change to a complete wetting case was found at all surfactant concentrations used. It has been observed that for all sponges, no matter the degree of saturation, they display a minimum contact angle after which the droplet is rapidly absorbed into the porous media.

A Monolayer Partitioning Scheme for Droplets of Surfactant Solutions.

Bulk‐surface partitioning of surface active species affects both cloud droplet activation by aerosol particles and heterogeneous atmospheric chemistry. Various approaches are given in the literature to capture this effect in atmospheric models. Here we present a simple, yet physically self‐contained, monolayer model for prediction of both composition and thickness of the surface layer of an aqueous droplet. The monolayer surface model is based on assuming a finite surface layer and mass balance of all species within the droplet. Model predictions are presented for binary and ternary aqueous surfactant model systems and compared to both experimental and model data from the literature and predictions using a common Gibbsian model approach. Deviations from Gibbsian surface thermodynamics due to volume constraints imposed by the finite monolayer lead to stronger predicted surface tension reduction at smaller droplet sizes with the monolayer model. Process dynamics of the presented monolayer model are also contrasted to other recently proposed approaches to treating surface partitioning in droplets, with different underlying assumptions.

Controlling Octagon-to-Square Wetting Interface Transition of Evaporating Sessile Droplet through Surfactant on Microtextured Surface.

Producing and maintaining specific liquid patterns during evaporation holds great potential for techniques of printing and coating. Here we report the control over the evolution of surfactant solution droplets on the micropyramid substrates during evaporation. The polygonal droplet shape is achieved during the drying rather than solely at the beginning. As the initial surfactant concentration is 0.04 mM, the droplet maintains its initial octagonal shape throughout the lifetime. Interestingly, the initial octagonal shape transforms into a square during the evaporation as the initial surfactant concentration reaches 0.8 mM. These findings can shed light on wetting pattern control for complex solutions required in various applications.

Evaporation kinetics of surfactant solution droplets on rice (Oryza sativa) leaves.

The dynamics of evaporating sessile droplets on hydrophilic or hydrophobic surfaces is widely studied, and many models for these processes have been developed based on experimental evidence. However, few research has been explored on the evaporation of sessile droplets of surfactant or pesticide solutions on target crop leaves. Thus, in this paper the impact of surfactant concentrations on contact angle, contact diameter, droplet height, and evolution of the droplets’ evaporative volume on rice leaf surfaces have been investigated. The results indicate that the evaporation kinetics of surfactant droplets on rice leaves were influenced by both the surfactant concentrations and the hydrophobicity of rice leaf surfaces. When the surfactant concentration is lower than the surfactant CMC (critical micelle concentration), the droplet evaporation time is much longer than that of the high surfactant concentration. This is due to the longer existence time of a narrow wedge region under the lower surfactant concentration, and such narrow wedge region further restricts the droplet evaporation. Besides, our experimental data are shown to roughly collapse onto theoretical curves based on the model presented by Popov. This study could supply theoretical data on the evaporation of the adjuvant or pesticide droplets for practical applications in agriculture.

Evaporation of Droplets of Surfactant Solutions

The simultaneous spreading and evaporation of droplets of aqueous trisiloxane (superspreader) solutions onto a hydrophobic substrate has been studied both experimentally, using a video-microscopy technique, and theoretically. The experiments have been carried out over a wide range of surfactant concentration, temperature, and relative humidity. Similar to pure liquids, four different stages have been observed: the initial one corresponds to spreading until the contact angle, θ, reaches the value of the static advancing contact angle, θad. Duration of this stage is rather short, and the evaporation during this stage can be neglected. The evaporation is essential during the next three stages. The next stage after the spreading, which is referred to herein as the first stage, takes place at constant perimeter and ends when θ reaches the static receding contact angle, θr. During the next, second stage, the perimeter decreases at constant contact angle θ = θr for surfactant concentration above the critical wetting concentration (CWC). The static receding contact angle decreases during the second stage for concentrations below CWC because the concentration increases due to the evaporation. During the final stage both the perimeter and the contact angle decrease. In what follows, we consider only the longest stages I and II. The developed theory predicts universal curves for the contact angle dependency on time during the first stage, and for the droplet perimeter on time during the second stage. A very good agreement between theory and experimental data has been found for the first stage of evaporation, and for the second stage for concentrations above CWC; however, some deviations were found for concentrations below CWC.

Actuation of Small Objects by using In-Plane Explosion Phenomena at Oil-Water Interfaces

An interfacial phenomenon has been investigated, which is caused by putting a droplet of surfactant solution at an oil-water interface. When a droplet of camphor solution is put into an oil-water interface, an “explosion phenomenon” occurs, in which the oil phase is broken with a phase transition within a short time, e.g. less than 1 s in a system. The explosion phenomenon is repeated by putting another droplet, due to the sublimation of camphor. By using the explosion phenomenon with camphor solution, an impeller with a diameter of 5 mm shows a rotational motion with a rotational speed of 1 000 rpm, in which a reaction force of a jet by camphor solution is utilized. The direction and magnitude of the force generated by the explosion phenomena is controllable by giving an asymmetry of wetting property to a solid surface of an object, even if the shape of the surface is symmetric. In order to acquire a directional force with the asymmetry of wetting property, the gap between the wetting-controlled solid surface and an oil-water interface must be in a range, e.g. ±0.7 mm for the present experimental system.

Influence of Dynamic Surface Tension on the Spreading of Surfactant Solution Droplets Impacting onto a Low-Surface-Energy Solid Substrate

We have investigated the impact of single droplets of various surfactant solutions on a low-surface-energy solid substrate using a high-frequency visualization technique (one picture every 100 μs). Whatever the surfactant, the drop spreads and retracts in about 1 s under the action of inertia and capillarity, respectively. During retraction, the capillary waves can be amplified and, in some cases, even yield droplet bouncing. Then, the droplet may slowly spread again due to gravity and the unbalanced capillary forces at the contact line between the droplet and the substrate. During the fast spreading process (2–3 ms), the droplet surface increases by almost one order of magnitude since its shape changes from a sphere to a flat pancake; this causes a strong deviation from thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which we have evaluated using a maximum bubble pressure apparatus. We have shown that droplet retraction is drastically influenced by the adsorption kinetics of the surfactant which limits the return to equilibrium surface tension.

PENETRATION OF OCTYLPHENOXY SURFACTANTS THROUGH ISOLATED TOMATO FRUIT CUTICLES

Despite the widespread use of surfactants to enhance the performance of foliar applied chemicals, the mechanisms for this enhancement are poorly understood. The penetration of surfactant per se through the cuticular membrane (CM) may play a pivotal role. Thus, we examined CM penetration by octylphenoxy surfactants (Triton X series) using a finite dose (Franz) diffusion cell. The effect of hydrophile length was studied using 14C surfactant (15.9 mm in 20 mm citrate buffer: pH 3.2) with 3, 9.5, 12, 16, and 40 ethylene oxide units per molecule (EO). One 5-μl droplet of surfactant solution was applied to the outer morphological surface of CM enzymatically isolated from mature tomato fruit. The inner CM surface remained in contact with stirred buffer at 25°C. The buffer was sampled periodically through a side portal over 648 h. Penetration curves (time vs. % penetrated) for all surfactants were characterized by three phases: lag, linear, and asymptotic. Lag: There was no effect of EO on the length of the lag phase (average 5 h) Linear: Steady state penetration (0.6 to 1.1% / h) was inversely related to log EO content. Asymptotic: About 70% of applied short EO (3 to 16) surfactants penetrated while 25% of the 40 EO penetrated in 648 h.

PENETRATION OF OCTYLPHENOXY SURFACTANTS THROUGH ISOLATED TOMATO FRUIT CUTICLES

Despite the widespread use of surfactants to enhance the performance of foliar applied chemicals, the mechanisms for this enhancement are poorly understood. The penetration of surfactant per se through the cuticular membrane (CM) may play a pivotal role. Thus, we examined CM penetration by octylphenoxy surfactants (Triton X series) using a finite dose (Franz) diffusion cell. The effect of hydrophile length was studied using 14C surfactant (15.9 mm in 20 mm citrate buffer: pH 3.2) with 3, 9.5, 12, 16, and 40 ethylene oxide units per molecule (EO). One 5-μl droplet of surfactant solution was applied to the outer morphological surface of CM enzymatically isolated from mature tomato fruit. The inner CM surface remained in contact with stirred buffer at 25°C. The buffer was sampled periodically through a side portal over 648 h. Penetration curves (time vs. % penetrated) for all surfactants were characterized by three phases: lag, linear, and asymptotic. Lag: There was no effect of EO on the length of the lag phase (average 5 h) Linear: Steady state penetration (0.6 to 1.1% / h) was inversely related to log EO content. Asymptotic: About 70% of applied short EO (3 to 16) surfactants penetrated while 25% of the 40 EO penetrated in 648 h.