Dewetting Research Articles

Tuning rheology and fire-fighting performance of protein-stabilized foam by actively switching the interfacial state of the liquid film

Biomolecular self-assembly has lately emerged as an intriguing method for creating stable gas-liquid dispersions with unique functional characteristics. In this work, protein-metal coordination complexes were designed as the stabilizer for generating ultrastable fire-fighting foam and creating interfacial architectures that were actively switched between “rigid” and “mobile” interfacial states of liquid films in response to changes in pH and bulk solution compositions (metal ions or alkyl polyglycosides). The reflected light interferometric technique was used to check interfacial states, and the foaming kinetics and rheological response of aqueous solution and liquid foam were investigated by dynamic surface tension tests and oscillatory rheology analysis. The results showed that liquid foams with mobile films with lower yield limits had a faster spreading rate to cover the burning oil, liquid foams with semi-rigid films cannot extinguish fires due to interfacial instability, and the enhanced rheology of the foam with rigid films established a robust and impenetrable barrier to effectively suppress fuel evaporation and combustion. A new correlation between interfacial properties and the fire-fighting performance of foam was proposed, which showed that the fire-extinguishing time of foam could be well correlated by the interfacial states or film lifetime rather than classical thermodynamics entry, spreading, and bridging coefficients (ESB coefficients).

Dilute oil-in-water emulsion initiates multiple holes formation during bubble bursting

The air bubble bursting dynamics involve intricate interfacial phenomena of millisecond timescale, which are of interest in many natural and industrial processes. The current study concerns bubble containing oil droplets with size comparable to bubble cap thickness. The assumption is that the dynamics of bubble bursting and aerosol formation will be modified as oil droplets interact with the air-water interface during drainage. Indeed, the experiments show that with the increase of the droplet spreading coefficient (S), the average film retraction speed decreases and the average film thickness increases at breakup. Notably, as S reaches 3.6 mN/m, a distinct multiple hole formation phenomenon has been observed for the first time during bubble bursting. The frequency of multiple hole formation increases with increasing S, and is as high as 22 holes per bursting event observed when S = 21.3 mN/m. The multiple holes evolve with time and form convoluted ligament structures that finally break up into much more aerosolized droplets. However, after normalization, all droplet size distribution is well predicated by a Gamma distribution.The results highlight the importance of the interfacial physiochemistry for the bubble cap rupture, regarding the wetting state between the oil droplets, air, water phase.

Synergistic effects between anionic surfactant SDS and hydrophilic silica nanoparticles in improving foam performance for foam flooding

In foam flooding, foams stabilized by conventional anionic surfactants are usually raptured in contacting with crude oil, which behaves as a strong defoamer. In this article the synergistic effects between anionic surfactant SDS and negatively charged hydrophilic silica nanoparticles in stabilization of aqueous foams were examined. The results show that in either deionized (DI) water at 25 °C or Daqing simulated connate (SC) water with a total salinity of 6,778 mg/L at 45 °C, in absence of oil adding low concentration of silica nanoparticles into the SDS aqueous solutions can improve the foam performance of SDS mainly by increasing foam stability, as indicated by a synergistic effect index Se(d/s) (ratio of the composite foaming index of dispersion to that of solution) of 2.7 to 2.8. In presence of oil the foam performance of SDS in SC water at 45 °C can still be improved as indicated by an increased Se(d/s) of 3.7, but the maximum composite forming index is catastrophically reduced from 3222 Ls in absence of oil to 470 Ls in presence of oil. As similarly charged nanoparticles the silica nanoparticles do not adsorb at gas/liquid or oil/water interface to stabilize Pickering foams or Pickering emulsions but can improve emulsification of the oil, and the emulsified oil droplets together with the silica nanoparticles distribute in aqueous lamellae enhancing liquid viscosity and slowing down the liquid drainage in the foams. However, this effect weakens at high surfactant concentration, and addition of silica nanoparticles does not significantly reduce the interfacial tensions involved. The Enter (E), Spreading (S) and Bridging (B) coefficients of the oil in the foam systems are therefore still positive. The defoaming action of the oil in this system is not eliminated by only partially inhibited by addition of silica nanoparticles.

Prince Rupert's Drop bouncing on high-speed moving superhydrophobic surfaces

Superhydrophobic surfaces have shown huge potential in diverse fields. However, the hydrodynamics of droplet impacting on moving superhydrophobic surfaces are largely unexplored, especially for surfaces with a high moving velocity. The knowledge gap in liquid-solid interaction between impacting droplets and high-speed moving superhydrophobic surfaces should be filled to better guide practical engineering applications. Here, a superhydrophobic surface is fabricated as the experimental surface by a chemical deposition-etching method and droplet impacting experiments on horizontally moving superhydrophobic surfaces (realized by impacting the edge of a rotating surface) are systematically conducted. The moving velocity of superhydrophobic surfaces reaches nearly 10.0 m/s, which is approximately three times the velocity in previous relevant studies. The results show that the droplet spreading is greatly enhanced when the surface velocity exceeds a critical value, and it is independent of the droplet Weber number (We). The droplet detaches from the surface in the shape of “Prince Rupert's Drop”, reducing the contact time by over 40%. The critical surface velocity is characterized by We/Wes = 0.17, where Wes is the Weber number of the moving surface. To reveal the dynamics of Prince Rupert's Drop bouncing, scaling laws of the maximum spreading coefficient and the contact time are developed, which agree well with experimental results.

Influence of Temperature-Guided SAM Growth on Wetting and Its Mass Transfer Models.

The formation and growth of self-assembled monolayers (SAMs) composed of amphiphiles have garnered significant attention due to their diverse technical applications. This article reports the findings of molecular dynamics simulations aimed at elucidating the intricate relationship between the wetting behavior of amphiphiles, specifically n-alkanols, and the growth of their SAMs on a mica surface under varying temperature conditions. The investigation quantifies the structural characteristics of the formed SAMs, including density profiles, in-plane radial distribution functions, order parameters, and end-to-end length distributions of n-alkanol molecules within the SAM. Thermodynamic properties, such as the second virial coefficient and excess entropy, are examined in relation to temperature and time. The growth of the SAM is assessed by analyzing characteristic time scales at different temperatures and in-plane diffusion of n-alkanol molecules and utilizing classical theories of mass transfer to quantify the growth rate as a function of temperature. These results are then correlated with changes in the contact angle and spreading coefficient of n-alkanol droplets on the mica surface over time, providing insights into the impact of SAM growth on the wetting behavior and the mass transfer model of such systems.

High-efficiency mechanism of enhancement of spreading performance of oil involving surfactant-laden oil droplet spreading over water surface

This study clarified the high-efficiency mechanism of enhancement of the spreading performance of oil by allowing a droplet of an organic solvent to spread over the surface of water with the addition of oil-soluble surfactants. Firstly, the effect of Span 85, Span 80, Span 20, octylphenol polyoxyethylene (4) ether (OP-4), and sodium dioctyl sulfosuccinate (AOT) on the interfacial parameters of an n-decane droplet was evaluated by measuring the surface tension and interfacial tension. Then the spreading coefficient was calculated to determine whether the surfactant-laden oil droplets could spread at the air–water interface. Moreover, the law governing the spreading of the surfactant-laden oil droplets over water was systematically studied. Furthermore, the characteristics of the dynamic behavior of the surfactants were investigated by molecular dynamics simulations to elucidate the spreading mechanism. The results of the experiments and simulations explained how the oil-soluble surfactants Span 85, Span 80, Span 20, OP-4, and AOT significantly improved the spreading performance of oil droplets over the water surface.

Cassie's Law Reformulated: Composite Surfaces from Superspreading to Superhydrophobic.

In 1948, Cassie provided an equation describing the wetting of a smooth, heterogeneous surface. He proposed that the cosine of the contact angle, θc, for a droplet on a composite surface could be predicted from a weighted average using the fractional surface areas, fi, of the cosines of contact angles of a droplet on the individual component surfaces, i.e., cos θc = f1 cos θ1 + f2 cos θ2. This was a generalization of an earlier equation for porous materials, which has recently proven fundamental to underpinning the theoretical description of wetting of superhydrophobic and superoleophobic surfaces. However, there has been little attention paid to what happens when a liquid exhibits complete wetting on one of the surface components. Here, we show that Cassie's equation can be reformulated using spreading coefficients. This reformulated equation is capable of describing composite surfaces where the individual surface components have negative (droplet state/partial wetting) or positive (film-forming/complete wetting) spreading coefficients. The original Cassie equation is then a special case when the combination of interfacial tensions results in a droplet state on the composite surface for which a contact angle can be defined. In the case of a composite surface created from a partial wetting (droplet state) surface and a complete wetting (film-forming) surface, there is a threshold surface area fraction at which a liquid on the composite surface transitions from a droplet to a film state. The applicability of this equation is demonstrated from literature data including data on mixed self-assembled monolayers on copper, silver, and gold surfaces that was regarded as definitive in establishing the validity of the Cassie equation. Finally, we discuss the implications of these ideas for super-liquid repellent surfaces.

Dynamics of nonlocal and local SIR diffusive epidemic model with free boundaries

This paper is concerned with nonlocal and local diffusive SIR epidemic model with free boundaries including convolution, which is natural extension of reaction diffusion systems with free boundary problems and local diffusions. The existence of unique global solution for this model is considered. Dichotomy of the spreading and vanishing is established. A spreading barrier line is found to determine whether the spreading of disease will fail finally. The spreading of disease will fail when it cannot spread across the spreading barrier line l∗, while it will be successful when it transcends over this barrier line. The results show that if the basic reproduction number R0<1, the spreading of disease will fail eventually, and if R0>1+d2μ2+α, the spreading of disease will get success finally. We also find that the spreading coefficients play important role in the spreading achievement. When 1+βθμ1<R0<1+d2μ2+α, the spreading coefficient decides whether the spreading of disease will be successful. It is shown that the spreading will be successful when the spreading coefficient is relatively big, while the spreading will fail if the spreading coefficient is relatively small.

Study on Dynamic Contact Behavior of Multi-Component Droplet and Dust Surface

The dynamic contact behavior between multi-component droplets and the surface of iron ore dust was taken as the research object, analysis of the maximum spreading coefficient, maximum acting diameter, maximum acting area, and maximum bouncing height of solid-liquid contact, from a microscopic perspective, using high-speed photography and image analysis and processing technology. The experimental results indicate that (1) with the particle size of dust particles decreases, the solid-liquid contact behavior sequentially manifests as spread immediately after broken, retraction, negative bounce, primary bounce, and secondary bounce. (2) When the surface tension of the droplets decreases from 55.5 to 34.8 mN/m, the maximum spreading diameter of the droplet has increased by 30% to 40%, the maximum bounce heights (coefficients) decreased by 100%, 57.14%, and 53.57%, respectively, the maximum spreading coefficient of the droplet exhibits no obvious pattern. (3) With decreasing droplet surface tension, the unidirectional acting diameter and the maximum acting area increase when the dust surface size is over 100 μm. When the surface particle size is less than 100 μm, there is no significant change in the unidirectional acting diameter and maximum acting area despite decreasing surface tension. Thus, droplet diffusion is mainly influenced by particle size. These findings contribute to enhancing the theory of water mist dust removal and improving dust removal efficiency.

Stochastic Modeling of the Epidemic Process Based On a Stage-Dependent Model with Non-Markov Constraints for Individuals

A continuous-discrete stochastic model of the epidemic process is presented. The model takes into account several stages of the development of an infectious disease, as well as the distributions of the durations of stay of individuals in these stages. The variables of the model are integer random variables that denote the quantity of individuals in cohorts, and sets of unique types of individuals that take into account the current state and history of stay of individuals in the stages of development of an infectious disease, distributions of durations of these stages are different from exponential or geometric. The results of an analytical and numerical research of the dynamics of the epidemic process are presented. The probabilities of infection eradication during a finite period of time are examined, depending on the numerical values of the infection spread coefficient and the distributions of the durations of the latent stage of the disease and the stage of preservation of immunity to infection.

Role of salts in fire extinguishing performance of aqueous film-forming foam (AFFF)

The active ingredients for aqueous film-forming foam (AFFF) are supplied as a concentrate, and the water used to dilute AFFF concentrates generally come from water sources around the fire site including seawater and river water. However, the effects of salt in water on fire-extinguishing performance of AFFF remain unclear. In this paper, we investigated the relationship between the interfacial properties, foam stability, and fire-extinguishing performance of three homemade AFFFs (nonionic AFFFAPG, amphoteric AFFFCAB and anionic AFFFSDS) and four commercial AFFFs. The seawater, firefighting water from different regions of China and deionized water were used to dilute AFFF concentrates. The results showed that AFFFAPG and AFFFCAB had a better performance than AFFFSDS in resisting the negative effects of salts on interfacial properties and foaming ability. In the fire-extinguishing tests, we found that dilution of AFFF with water samples containing high concentrations of Ca2+ and Mg2+ would weaken the fire-extinguishing performance. The traditional indicators characterizing the properties of foam (surface tension, liquid drainage rate, and spreading coefficient) could not explain the trend of fire extinguishing performance. The stability of AFFF in the presence of oil could be used to explain the difference in fire-extinguishing time of AFFF.

Emulgel: A Novel Technique for Transdermal Drug Delivery

Topical drug delivery is obtained chiefly for the local dermatological action, but new technologies recently enhanced its systemic effect. They are generally applied for antiseptics, antifungal agents, skin emollients, and protectants. The activity of topical preparation confides in various factors such as drug solubility, lipophilicity, contact time to the skin, and permeability. Typically topical agents like lotions, ointments, creams, gel are associated with drawbacks like stability problems, stickiness, poor permeability, poor absorption, lesser spreading coefficient, irritation, allergic reactions, and difficulty absorbing large molecules. The new concept of Emulgel has been introduced Emulgel. Emulgel systems are currently of attention to pharmaceutical scientists because of their substantial potential to act as drug delivery vehicles by incorporating a broad range of drug molecules. These are either an emulsion of water in oil type or oil in water, gelled by blending it with a gelling agent. Incorporating the emulsion into gel makes it a dual control release system and also increases its stability. The overview gives the ideal properties, formation, and evaluation of emulgels.

Influence of Binder on Surface Properties and Coating Effect of TKX-50 Explosive

In order to study the surface interaction between binder and TKX-50, fluororubber F2603, ETPE, and BR were adhered to the surface of TKX-50 by the direct solvent volatilization method. The spreading coefficient and adhesion work between the three adhesives and TKX-50 were analyzed by a surficial performance test. The coating effect and safety of TKX-50 coated by three binders were discussed by XPS, SEM, and mechanical sensitivity. The results showed that F2603, ETPE, and BR were adhered to the surface of TKX-50 to form efficient spreading and bonding. The coating effect of F2603 on the TKX-50 was better than that of ETPE and BR, which improved the mechanical sensitivity of TKX-50. The results show that some binders could have positive effects on improving the safety of TKX-50.

Interfacial Adhesion Property of Asphalt Binder with Calcium Alginate Carrier of Asphalt Rejuvenator.

Recently calcium alginate has been successfully applied to encapsulate asphalt rejuvenator, which can protect asphalt rejuvenator from early leakage and release asphalt rejuvenator when triggered by specific factors such as cracks. The interfacial adhesion property of asphalt binder with calcium alginate carrier is of great importance to its actual performance. In this paper, the molecular model of the interface region between asphalt binder and calcium alginate was established, and molecular dynamics simulations were performed on it to investigate the molecular interaction at the interface region. By extracting and processing the data during the simulation process, the interfacial adhesion behavior was expounded using the spreading coefficient (S), permeation depth and permeation degree. Furthermore, the interfacial adhesion strength was evaluated by adopting the interfacial adhesion work. Results showed that the value of S was greater than 0, implying that asphalt binder could wet the surface of calcium alginate. Saturate had the highest value of permeation degree, followed by resin, aromatic and asphaltene. However, asphalt binder could not infiltrate into the interior of TiO2, only accumulating and spreading on the surface of TiO2. The interfacial adhesion work of unaged and aged asphalt binder to calcium alginate was -114.18 mJ/m2 and -186.37 mJ/m2, respectively, similar to that of asphalt-aggregate interface. The van der Waals interactions contributed the most to the formation of the interfacial adhesion strength. In addition, a certain degree aging of asphalt binder and addition of titanium dioxide in the calcium alginate carrier were helpful to enhance the interfacial adhesion strength.

Study on wetting mechanism of nonionic silicone surfactant on coal dust

Coal dust disasters are serious in coal mining. The use of nonionic surfactants can effectively improve the wettability of coal dust and reduce the content of suspended coal dust in the air. For the problem of low wettability of ordinary surfactants, this paper selects silicone surfactants with high surface activity and low surface tension to improve the wetting ability of coal dust. To explore the wettability of nonionic silicone surfactants on coal dust, the effects of six nonionic silicone surfactants on the wettability of coal dust surfaces were studied by experiments. The test objects were four kinds of coal samples with different metamorphic degrees. The surface tension, wetting time, and contact angle experiments were carried out, and the critical micelle concentration and the expansion coefficient of the coal surface were calculated. The wetting time of the compound solution was measured to verify the synergistic effect of the compound solution. The results show that: 6 # has the best wetting effect on coal dust, followed by 4 # and 2 #; The order of surface tension is: 1 # < 3 # < 4 # < 6 # < 5 # < 2 #, the surface tension of 1 # is the lowest (19.962 mN/m); 1 # and 4 # are easier to spread on the surface of coal dust, the spreading coefficient of coking coal is the largest and the contact angle is the smallest, which is 18.8°. The 4 # and 6 # with a mass ratio of 8:2 were compounded. The compound surfactant solution had a significant synergistic effect. Compared with the monomer surfactant solution, the wettability of long-flame coal and coking coal increased by 15.14% and 10.00%, respectively. The results of this study can provide reference and experimental support for the development of high-efficiency dust suppressants based on silicone surfactants.

Effect of Sodium Carboxymethyl Cellulose on the Dynamic Wetting Characteristics of the Dust Suppression Droplet Impacting the Coal Surface.

The dynamic wetting behavior of droplets impacting the coal surface directly affects the efficient application of water-based dust suppression materials in coal-related industrial production. In this paper, ultrapure water, Tween-80, and sodium carboxymethyl cellulose are taken as the research objects. Using high-speed photography technology, the spreading, oscillation process, and splash morphology of many kinds of droplets during impacting the coal surface are captured. The effects of viscosity, surface tension, and impact velocity on dynamic wetting characteristics were studied. The results show that with the decrease of surface tension, the retraction and oscillation of droplets are significantly reduced. For the same kind of droplets, the greater the impact velocity, the faster the droplet spread, and the dimensionless maximum spreading coefficient (βmax) and dimensionless steady-state spreading coefficient (βe) of droplets are bigger. With the increase of velocity, the time for different kinds of droplets to reach the βmax increases. At the same impact velocity, βmax and βe of droplets (0.2% Tween-80 + 0.1% sodium carboxymethyl cellulose) are the largest, indicating that adding a small amount of sodium carboxymethyl cellulose can promote droplet spreading. With the increase of sodium carboxymethyl cellulose content, βmax and βe decreased gradually. The results have a great significance to the research, development, and scientific utilization of water-soluble polymer dust inhibitors.

Effect of Wettability on the Collision Behavior of Acoustically Excited Droplets.

Acoustic droplet ejection (ADE) is a noncontact technique for micro-liquid handling (usually nanoliters or picoliters) that is not restricted by nozzles and enables high-throughput liquid dispensing without sacrificing precision. It is widely regarded as the most advanced solution for liquid handling in large-scale drug screening. Stable coalescence of the acoustically excited droplets on the target substrate is a fundamental requirement during the application of the ADE system. However, it is challenging to investigate the collision behavior of nanoliter droplets flying upward during the ADE. In particular, the dependence of the droplet's collision behavior on substrate wettability and droplet velocity has yet to be thoroughly analyzed. In this paper, the kinetic processes of binary droplet collisions were investigated experimentally for different wettability substrate surfaces. Four states occur as the droplet collision velocity increases: coalescence after minor deformation, complete rebound, coalescence during rebound, and direct coalescence. For the hydrophilic substrate, there are wider ranges of Weber number (We) and Reynolds number (Re) in the complete rebound state. And with the decrease of the substrate wettability, the critical Weber and Reynolds numbers for the coalescence during rebound and the direct coalescence decrease. It is further revealed that the hydrophilic substrate is susceptible to droplet rebound because the sessile droplet has a larger radius of curvature and the viscous energy dissipation is greater. Besides, the prediction model of the maximum spreading diameter was established by modifying the droplet morphology in the complete rebound state. It is found that, under the same Weber and Reynolds numbers, droplet collisions on the hydrophilic substrate achieve a smaller maximum spreading coefficient and greater viscous energy dissipation, so the hydrophilic substrate is prone to droplet bounce.

Dynamic characteristics of droplet eccentrically impacting on the surface of a hydrophilic cylinder

Droplet impact widely exists in a variety of industrial application, and its dynamic characteristics have a significant effect on the system efficiency. In this study, the behavior characteristics of droplets impacting hydrophilic wires are carried out by combining experiments and numerical simulations. Ignore the airflow disturbance in the environment, the effects of eccentricity, Weber number and Reynolds number on droplet morphology evolution under the condition of fixed size ratio of droplet/cylinder are analyzed, and the flow characteristics of central and eccentric impact of droplets are compared. The results show that the eccentricity, the Weber number and the Reynolds number of the droplets have significant effects on the dynamic characteristics of the droplets. With the increase of the eccentricity, the maximum spreading coefficient gradually increases, and there are four typical modes: symmetrical spreading-wrapping, asymmetric spreading-wrapping, unilateral wetting and partial wetting. When the Weber number is 51, the droplet spreading and retracting behavior is obvious. With the increase of Weber number, the droplet impact spreading velocity is accelerated, and the maximum spreading coefficient is gradually increased. With the increase of the eccentricity or the decrease of the Reynolds number, the instability of the spreading of the liquid film increases, and the droplet is more difficult to be caught by the wire.

Effects of ridge parameters on axial spreading of droplet impact on superhydrophobic surfaces

Superhydrophobic surfaces decorated with macrostructures have presented remarkable potential in diverse engineering fields, such as aircraft anti-icing. Understanding the effects of the structure shape and size on droplet dynamics is crucial to the design and application of surfaces. Herein, we investigate the maximum axial spreading for droplets impacting on ridged superhydrophobic surfaces with varied ridge shapes and sizes. We propose a mathematical formula to describe the structure shape with profiles quantified by the shape factor, which is easily applied to structure-related studies. The effects of ridge shape and size on the maximum axial spreading coefficient are clarified. The axial spreading of droplets is inhibited by the ridge due to the outward flow of liquid above the ridge tip. The maximum axial spreading coefficient reduces when the ridge becomes sharper, which can be achieved by increasing the shape factor or the ridge height–width ratio. The complex effect of the ridge–droplet size ratio is divided into two regimes according to the shape factor. Furthermore, a prediction correlation of the maximum axial spreading coefficient is established, which involves the coupled effects of all parameters, agreeing well with experimental and simulation results.

Coalescence dynamics of droplets collision on substrates with different contact angles

Droplet collision coalescence is widely encountered in the petroleum industry, chemical industry, and agricultural applications. In this study, the dynamic behavior of the coalescence process of a free-falling droplet impacting a sessile droplet on substrates with different wettabilities is investigated via high-speed photography. The influences of the substrate contact angle (α) and the Weber number (We) of the droplet collision were analyzed. Based on the experiments, the process of droplet collision coalescence was divided into four stages: coalescence, spreading, retraction, and oscillation. The maximum spreading diameters, maximum jet heights, spreading velocities, and retraction velocities of droplets with different values of the substrate contact angle and We were obtained. The dynamic mechanism of different collision coalescence stages was revealed by the interconversion of inertial forces, surface tension, and wall adhesion forces. The relationship between the dimensionless spreading coefficient and the dimensionless time was established for different values of the collision We and contact angle α based on the experimental results. The effects of We and α on the coalescence process of the droplets were obtained by analysis of the energy conversion.