Droplet Adhesion Research Articles

Effect of cement on the demulsifying behavior of over-stabilized asphalt emulsion during mixing

Over-stabilized asphalt emulsion is normally used in cement asphalt emulsion (CA) composites. The mixing stability of asphalt emulsion with cement is vital to the workability and mechanical properties of CA composites. However, the effect of cement on the demulsifying behavior of asphalt emulsion during mixing is not clearly understood. To reveal the demulsifying behavior, effect of pH value and Ca2+ on the rheological properties and mixing stability of over-stabilized asphalt emulsion are studied. Results indicate that cationic emulsions have no obvious change in the tested pH value and Ca2+ concentration range. Ca2+ ion is very harmful to the chemical stability of anionic emulsion. There are three stages in the demulsifying process of pure emulsion: (1) small asphalt droplet coalescence along with trapped water; (2) trapped water diffuses out from coalesced asphalt droplets; (3) complete demulsification. There are two demulsifying behaviors when asphalt emulsion is mixed with fine solid. One is asphalt droplet coalescence, which is dominant in the mixing with limestone powder. The other is the direct droplet adhesion on solid surface, which is dominated in the mixing with cement. This difference is due to that the adsorption ability of cement is much stronger than that of limestone powder. The ions of OH− and Ca2+ released by cement hydration cannot greatly destabilize asphalt emulsion. However, emulsions with better resistance to the invasion of OH− and Ca2+ still have a better mixing stability. Therefore, reducing the adsorption ability of cement and increasing the resistance of emulsions to hydrate ions can improve the mixing stability of asphalt emulsion with cement.

Adhesion of mechanically and chemically dispersed crude oil droplets to eggs of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus)

Crude oil accidentally spilled into the marine environment undergoes natural weathering processes that result in oil components being dissolved into the water column or present in particulate form as dispersed oil droplets. Oil components dissolved in seawater are typically considered as more bioavailable to pelagic marine organisms and the main driver of crude oil toxicity, however, recent studies indicate that oil droplets may also contribute. The adhesion of crude oil droplets onto the eggs of pelagic fish species may cause enhanced transfer of oil components via the egg surface causing toxicity during the sensitive embryonic developmental stage. In the current study, we utilized an oil droplet dispersion generator to generate defined oil droplets sizes/concentrations and exposed Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) to investigate if the potential for dispersed oil droplets to adhere onto the surface of eggs was species-dependent. The influence of a commercial chemical dispersant on the adhesion process was also studied. A key finding was that the adhesion of oil droplets was significantly higher for haddock than cod, highlighting key differences and exposure risks between the two species. Scanning electron microscopy indicates that the differences in oil droplet adhesion may be driven by the surface morphology of the eggs. Another important finding was that the adhesion capacity of oil droplets to fish eggs is significantly reduced (cod 37.3%, haddock 41.7%) in the presence of the chemical dispersant.

Magnetically Responsive Superhydrophobic Surface: In Situ Reversible Switching of Water Droplet Wettability and Adhesion for Droplet Manipulation.

A smart, magnetically responsive superhydrophobic surface was facilely prepared by combining spray coating and magnetic-field-directed self-assembly. The surface comprised a dense array of magnetorheological elastomer micropillars (MREMPs). Benefitting from the magnetic field-stiffening effect of the MREMPs, the surface exhibited reversible switching of the wettability and adhesion that was responsive to an on/off magnetic field. The wettability and adhesion properties of the surfaces with MREMPs were investigated under different magnetic fields. The results revealed that the adhesion force and sliding behaviors of these surfaces were strongly dependent on the intensity of the applied magnetic field and the mixing ratio of poly(dimethylsiloxane) (PDMS), iron particles, and solvent (in solution) used for preparation of the magnetically responsive superhydrophobic surfaces. The adhesion transition was attributed to the tunable mechanical properties of the MREMPs, which was easily controlled by an external magnetic field. It was also demonstrated that the magnetically responsive superhydrophobic surface can be used as a "mechanical hand" for no-loss liquid droplet transportation. This magnetically responsive superhydrophobic surface not only provides a novel interface for microfluidic control and droplet transportation, but also opens up new avenues for achieving smart liquid-repellent skin, programmable fluid collection and transport, and smart microfluidic devices.

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure.

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion co-flow device to generate uniform high-viscosity droplets in a low-viscosity fluid. The microfluidic capillary device has a common co-flow structure with its exit connecting to a wider tube. Elongated droplets of the low-viscosity fluid are first encapsulated by the high-viscosity fluid in the co-flow structure. As the elongated low-viscosity droplets flow through the exit, which is treated to be wetted by the low-viscosity fluid, phase inversion is then induced by the adhesion of the low viscosity droplets to the tip of the exit, which results in the subsequent inverse encapsulation of the high-viscosity fluid. The size of the resultant high-viscosity droplets can be adjusted by changing the flow rate ratio of the low-viscosity fluid to the high-viscosity fluid. We demonstrate several typical examples of the generation of high-viscosity droplets with a viscosity up to 11.9 Pas, such as glycerol, honey, starch, and polymer solution. The method provides a simple and straightforward approach to generate monodisperse high-viscosity droplets, which may be used in a variety of droplet-based applications, such as materials synthesis, drug delivery, cell assay, bioengineering, and food engineering.

Tuning orb spider glycoprotein glue performance to habitat humidity

Orb-weaving spiders use adhesive threads to delay the escape of insects from their webs until the spiders can locate and subdue the insects. These viscous threads are spun as paired flagelliform axial fibers coated by a cylinder of solution derived from the aggregate glands. As low molecular mass compounds (LMMCs) in the aggregate solution attract atmospheric moisture, the enlarging cylinder becomes unstable and divides into droplets. Within each droplet an adhesive glycoprotein core condenses. The plasticity and axial line extensibility of the glycoproteins are maintained by hygroscopic LMMCs. These compounds cause droplet volume to track changes in humidity and glycoprotein viscosity to vary approximately 1000-fold over the course of a day. Natural selection has tuned the performance of glycoprotein cores to the humidity of a species' foraging environment by altering the composition of its LMMCs. Thus, species from low-humidity habits have more hygroscopic threads than those from humid forests. However, at their respective foraging humidities, these species' glycoproteins have remarkably similar viscosities, ensuring optimal droplet adhesion by balancing glycoprotein adhesion and cohesion. Optimal viscosity is also essential for integrating the adhesion force of multiple droplets. As force is transferred to a thread's support line, extending droplets draw it into a parabolic configuration, implementing a suspension bridge mechanism that sums the adhesive force generated over the thread span. Thus, viscous capture threads extend an orb spider's phenotype as a highly integrated complex of large proteins and small molecules that function as a self-assembling, highly tuned, environmentally responsive, adhesive biomaterial. Understanding the synergistic role of chemistry and design in spider adhesives, particularly the ability to stick in wet conditions, provides insight in designing synthetic adhesives for biomedical applications.

Influence of Using Recycled Polyurethane Particles as a Filler on Properties of Polyurethane Adhesives for Gluing of Wood

Fillers can greatly affect the properties of adhesives, and this research aimed at enhancing the performance of adhesives by using different concentrations of fillers. This paper describes the influence of using recycled polyurethane particles (powder of particle size from 10 µm to 50 µm) as a filler on some properties of polyurethane adhesives for the gluing of wood. Two kinds of one-component, moisture-curing polyurethane adhesives were used. The observed properties were the contact angle between the wood and adhesive droplet, and the strength of bonded joints (shear strength). From the results, it was concluded that the contact angle increased with an increase in filler (in the entire observed range from 0% to 15% filler). The content of filler also affected the strength of bonded joints and their thermal stability. The strength of bonded joints decreased with an increase in filler content when samples were conditioned in cold water. However, the strength of bonded joints increased with an increase in filler content when samples were boiled in water.

Mimicking Cell Pinocytosis: Lipid Vesicles Engulfment of Oil-in-Water Droplets

Pinocytosis, a type of endocytosis, is a process by which extracellular liquid droplets are taken up by cells. It involves droplet adhesion and engulfment by the membrane. We develop a synthetic system that reproduces the essential steps of pinocytosis at the level of a single interacting pair - the membrane and the droplet. The system consists of an engulfing giant unilamellar vesicle (GUV) and an engulfed polymer-stabilized oil-in-water emulsion droplet. The interaction between the GUV and the droplet is mediated by magnesium ions and is reversed by EDTA or Triton X-100. Adhesion energy |W|, measured from the contact angle between a GUV adhering to a droplet, either freestanding or manipulated with micropipettes, is affected by the magnesium concentration. In the weak adhesion regime (|W|∼0.4 mJ/m2, ≤ 0.5 mM Mg2+), GUVs adhere to the droplets and deform. Interaction is reversed by retracting the pipette-held GUV. In the strong adhesion regime (γ∼1.2 mJ/m2, 0.5 > mM Mg2+), the GUV completely engulfs the droplet and the interaction is irreversible. The droplet becomes wrapped by a single bilayer in a process that increases membrane tension and leads to vesicle poration. Both adhesion and engulfment lead to a decrease in lipid diffusion coefficient from 7 μm2/s on free-standing bilayers to 1 μm2/s on the droplet-supported bilayer. Interestingly, the contact line of the membrane-droplet adhesion area is often connected to many lipid nanotubes. The developed synthetic pinocytosis system of GUV-droplet pairs allows detailed investigation of the endocytosis processes of natural cells but with reduced complexity. Future surface modifications of the membrane/droplet interaction have the potential to mimic specific pathogen-recognition interactions. This work is part of the MaxSynBio consortium which is jointly funded by the Federal Ministry of Education and Research of Germany and the Max Planck Society.

Adhesion of emulsified oil droplets to hydrophilic and hydrophobic surfaces - effect of surfactant charge, surfactant concentration and ionic strength.

Adhesion of emulsified oil droplets to a surface plays an important role in processes such as crossflow membrane filtration, where the oil causes fouling. We present a novel technique, in which we study oil droplets on a model surface in a flow cell under shear force to determine the adhesive force between droplets and surface. We prepared an emulsion of hexadecane and used hydrophilic and hydrophobic glass slides as model surfaces. Different surfactants were used as emulsifiers: negatively charged sodium dodecyl sulphate (SDS), positively charged hexadecyltrimethylammonium bromide (CTAB) and nonionic Triton X-100. We evaluate the role of the surfactant, the glass surface properties and the ionic strength of the emulsion. We found a minimum in the adhesion force between droplets and surface as a function of surfactant concentration. The charged surfactants cause a lower droplet adhesion compared to the nonionic surfactant. The flow cell technique presented here proved to be very useful in understanding the interaction between oil droplets and a surface.

Electrodeposited Zn for Water-Repellent Coatings

We show that mildly alkaline electrolytes can be used to produce Zn coatings that improve the water repellent properties of stainless steel. Optimal Zn deposits were prepared under potentiostatic conditions from electrolytes that contained ZnCl2, NH4Cl, and a surfactant (polyethyleneimine). After deposition, the Zn electrodeposit was capped with stearic acid to prevent oxidation and to provide a lower surface energy. The capped electrodeposits display an impressive degree of water repellency, including extremely poor water droplet adhesion. We discuss the range of deposition parameters (electrolyte composition, pH, and applied potential) that produce the best water-repellent electrodeposits.

Influences on Shotcrete Rebound from Walls with Random Roughness

Concrete slurry can be sprayed on walls for reinforcement; however, there is a certain amount of rebound which is hazardous, lowers production quality, and wastes material. To investigate this problem, we studied single slurry droplets at the mesoscopic level. We deduced the factors influencing droplet spreading and wall adhesion to create models of shotcrete rebound. Then, a numerical simulation orthogonal experiment investigating droplet‐wall impacts was performed. The relationship between the spreading coefficient and each influencing factor is discussed, and numerical models are presented. Finally, the obtained models are verified by physical experiments. The results show that the spreading coefficient can be used to better characterize the effect of slurry droplet adhesion to walls. Modeled and experimentally observed droplet‐wall impacts showed good consistency. The influence of each factor on the spreading coefficient was determined in the following order of strength: droplet velocity and viscosity, wall roughness, and surface tension. The spreading coefficient increases with velocity, decreases with viscosity and roughness, and increases first and then decreases with surface tension. This study improves the fluid dynamics‐based theory of multiphase flow in concrete slurry and provides a theoretical basis for mitigating shotcrete rebound.

Mapping microscale wetting variations on biological and synthetic water-repellent surfaces

Droplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10 µm and three orders of magnitude better force sensitivity than current tensiometers. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform.

Surfactant-induced enhancement of droplet adhesion in superhydrophobic soybean (Glycine max L.) leaves

This study performed with soybean (Glycine max L.), one of the most important crops for human and animal nutrition, demonstrates that changes in the leaf surface structure can increase the adhesion of applied droplets, even on superhydrophobic leaves, to reduce undesirable soil contamination by roll-off of agrochemical formulations from the plant surfaces. The wettability and morphology of soybean (Glycine max L.) leaf surfaces before and after treatment with six different surfactants (Agnique® SBO10 and five variations of nonionic surfactants) have been investigated. The leaf surface structures show a hierarchical organization, built up by convex epidermal cells (microstructure) and superimposed epicuticular platelet-shaped wax crystals (micro- to nanostructure). Chemical analysis of the epicuticular wax showed that 1-triacontanol (C30H61OH) is the main wax component of the soybean leaf surfaces. A water contact angle (CA) of 162.4° (σ = 3.6°) and tilting angle (TA) of 20.9° (σ = 10.0°) were found. Adherence of pure water droplets on the superhydrophobic leaves is supported by the hydrophilic hairs on the leaves. Agnique® SBO10 and the nonionic surfactant XP ED 75 increased the droplet adhesion and caused an increase of the TA from 20.9° to 85° and 90°, respectively. Scanning electron microscopy showed that surfactants with a hydrophilic–lipophilic balance value below 10 caused a size reduction of the epicuticular wax structures and a change from Cassie–Baxter wetting to an intermediate wetting regime with an increase of droplet adhesion.

Precision moulding of biomimetic disposable chips for droplet-based applications

In this study, we present a biomimetic approach to improve the stability and reproducibility of droplet generation processes and to reduce the adhesion of aqueous droplets to channel surfaces of microfluidic polymer chips. The hierarchical structure of the lotus leaf was used as a template for a partial laser structuring of the moulds that were used for casting the polymer chips. The hydrophobic wax layer of the lotus leaf was technologically replicated by coating the polymer chips using a plasma deposition process. The resulting microfluidic polymer chip surfaces reveal a topography and a surface free energy similar to those of the lotus leaf. Subsequent droplet-based microfluidic experiments were performed using a 2D flow focussing set-up. Droplets from both, serum-supplemented cell culture medium and anticoagulated human whole blood, could be generated stably and reproducibly using a fluorocarbon as continuous phase. The presented results illustrate the application potential of the lotus-leaf-like polymer chips in life sciences, e.g. in the field of personalised medicine.

Enhanced Coalescence-Induced Droplet-Jumping on Nanostructured Superhydrophobic Surfaces in the Absence of Microstructures.

Superhydrophobic surfaces are receiving increasing attention due to the enhanced condensation heat transfer, self-cleaning, and anti-icing properties by easing droplet self-removal. Despite the extensive research carried out on this topic, the presence or absence of microstructures on droplet adhesion during condensation has not been fully addressed yet. In this work we, therefore, study the condensation behavior on engineered superhydrophobic copper oxide surfaces with different structural finishes. More specifically, we investigate the coalescence-induced droplet-jumping performance on superhydrophobic surfaces with structures varying from the micro- to the nanoscale. The different structural roughness is possible due to the specific etching parameters adopted during the facile low-cost dual-scale fabrication process. A custom-built optical microscopy setup inside a temperature and relative humidity controlled environmental chamber was used for the experimental observations. By varying the structural roughness, from the micro- to the nanoscale, important differences on the number of droplets involved in the jumps, on the frequency of the jumps, and on the size distribution of the jumping droplets were found. In the absence of microstructures, we report an enhancement of the droplet-jumping performance of small droplets with sizes in the same order of magnitude as the microstructures. Microstructures induce further droplet adhesion, act as a structural barrier for the coalescence between droplets growing on the same microstructure, and cause the droplet angular deviation from the main surface normal. As a consequence, upon coalescence, there is a decrease in the net momentum in the out-of-plane direction, and the jump does not ensue. We demonstrate that the absence of microstructures has therefore a positive impact on the coalescence-induced droplet-jumping of micrometer droplets for antifogging, anti-icing, and condensation heat transfer applications.

Droplet heat transfer on micro-post arrays: Effect of droplet size on droplet thermal characteristics

Heat transfer towards a water droplet from hydrophobic micro-post array surface is considered while mimicking the environmental temperatures. Micro-post arrays are created on a silicon wafer surface via lithography technique. The textured surfaces are replicated by polydimethylsiloxane (PDMS) to achieve an optical transmittance. The droplet adhesion on micro-post array surface is presented and the influence of droplet size on the heat transfer and droplet internal flow characteristics is examined. The flow predictions are validated via the particle image velocimetry data. It is found that adhesion force between the water droplet and the micro-post arrays surface depends on the geometric size and the orientation of the micro-post arrays on the surface. Temperature and flow fields are influenced by the droplet size. The Nusselt and the Bond numbers increase with the droplet volume; however, the Bond number remains less than unity indicating that the Marangoni current dominates over the buoyancy current in the droplet. The Nusselt number attains larger values for micro-post array surface than that of the plain surface. This is because of temperature and velocity oscillations along the contact lines at the droplet bottom due to the pitches of the micro-post arrays.

Anti-icing properties of superhydrophobic stainless steel mesh at subzero temperatures

Superhydrophobic stainless steel meshes were designed by using nanosecond laser texturing followed by chemisorption of fluorooxysilanes from solution. The results of the study of anti-icing properties in contact with a humid atmosphere and saltwater and under a cyclic icing/melting process are presented for two types of superhydrophobic meshes. The essential anti-icing performance of fabricated superhydrophobic meshes in the conditions of mild winter season was proved by experiments of different types. Thus, low adhesion of supercooled saltwater droplets to the mesh surface at the studied subzero temperatures was demonstrated. Moreover, the preservation of a supercooled liquid state for saltwater droplets deposited onto both types of samples at temperatures of up to T = −10°C for tens of hours was detected, indicating good anti-icing properties of the coatings. Finally, the chemical stability in long-term contact with an aqueous phase and mechanical stability against stresses accompanying cyclic water freezing/melting shows the good potential of the developed coatings for outdoor winter applications.

Creation of wettability contrast patterns on metallic surfaces via pen drawn masks

Micropatterned surfaces with wettability contrast have attracted considerable attention due to potential applications in 2D microfluidics, bioassays, and water harvesting. A simple method to develop wettability contrast patterns on metallic surfaces by using a commercial marker is described. A marker-drawn ink pattern on a copper surface displays chemical resistance to an aqueous solution of sodium bicarbonate and ammonium persulfate, thereby enabling selective nanowire growth in areas where ink is absent. Subsequent ink removal by an organic solvent followed by fluorocarbon film deposition yields a stable hydrophobic/super-hydrophobic patterned copper surface. Using this approach, hydrophobic dot and line patterns were constructed. The adhesion force of water droplets to the dots was controlled by adjusting pattern size, thus enabling controlled droplet transfer between two surfaces. Anisotropy of water droplet adhesion to line patterns can serve as a basis for directional control of water droplet motion. This general approach has also been employed to generate wettability contrast on aluminum surfaces, thereby demonstrating versatility. Due to its simplicity, low cost, and virtual independence of solid surface material, ink marker pens can be employed to create wettability patterns for a variety of applications, in fields as diverse as biomedicine and energy.

Forming Sticky Droplets from Slippery Polymer Zwitterions.

Polymer zwitterions are generally regarded as hydrophilic and repellant or "slippery" materials. Here, a case is described in which the polymer zwitterion structure is tailored to decrease water solubility, stabilize emulsion droplets, and promote interdroplet adhesion. Harnessing the upper critical solution temperature of sulfonium- and ammonium-based polymer zwitterions in water, adhesive droplets are prepared by adding organic solvent to an aqueous polymer solution at elevated temperature, followed by agitation to induce emulsification. Droplet aggregation is observed as the mixture cools. Variation of salt concentration, temperature, polymer concentration, and polymer structure modulates these interdroplet interactions, resulting in distinct changes in emulsion stability and fluidity. Under attractive conditions, emulsions encapsulating 50-75% oil undergo gelation. By contrast, emulsions prepared under conditions where droplets are nonadhesive remain fluid and, for oil fractions exceeding 0.6, coalescence is observed. The uniquely reactive nature of the selected zwitterions allows their in situ modification and affords a route to chemically trigger deaggregation and droplet dispersion. Finally, experiments performed in a microfluidic device, in which droplets are formed under conditions that either promote or suppress adhesion, confirm the salt-responsive character of these emulsions and the persistence of adhesive interdroplet interactions under flow.

Viscoelastic, thermo-mechanical and environmental properties of composites based on polypropylene/poly(lactic acid) blend and copper modified nanoclay

Poly(lactic acid) (PLA)/polypropylene (PP) blends composites were prepared by incorporating 3 wt.% of copper modified montmorillonite (MMT-Cu2+), obtained using cation exchange in a CuSO4 solution, and 10 wt.% of polypropylene-graft-maleic anhydride (PP-g-MA) as a compatibilizer then varying the PLA content until 50 wt.%. These materials were subjected to several investigations such as X-rays diffraction, dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and tensile and environmental tests. The DMTA analysis showed that the glassy PLA high stiffness and the PP crystalline phase compensate the decrease in the storage modulus occurring during the PP and PLA glass transitions, respectively. The variations of tan δ revealed no changes on the PP and PLA phases glass transitions temperatures which indicate the immiscibility of the two polymers, as supported by DSC analysis. Blends composites SEM micrographs stated the immiscibility of the system resulting in the poor adhesion of the PLA droplets to the PP matrix. Also, the blends composites exhibited intermediate tensile properties between those of PP and PLA. The incorporation of MMT-Cu2+ to the (50/50) PP/PLA blend accentuated its aptitude to water absorption and ensured an efficient antimicrobial activity over a satisfactorily long period of around six months.

Water Droplet Adhesion on Hydrophobic Surfaces: Influence of Droplet Size and Inclination Angle of Surface on Adhesion Force

Adhesion of various size sessile droplets on the hydrophobic surfaces is considered, and the moment generated about the locus of the droplet meniscus is determined for several inclination angles of hydrophobic surface. An experiment is designed to examine the influence of inclination of hydrophobic surface on the water droplet behavior. The flow field generated inside the droplet is simulated to predict the flow acceleration and its effects on adhesion force. Simulations are repeated for different inclination angles of hydrophobic surface. The flow predictions are validated through the experimental data. It is found that the moment about the locus of droplet meniscus increases with increasing inclination angle, which is more pronounced for the large volume water droplets, such as ∀ = 45 μL; however, further increase of inclination angle lowers the moment because of significant change of the location of the line of action of the total force during the excessive body deformation of the droplet. The flow field developed inside the droplet forms a circulation cell, and the orientation and size of the circulation cell change with droplet volume, which becomes significant at high inclination angles. The flow acceleration inside the droplet does not have significant contribution to the overall force generated on the droplet during the inclination of the hydrophobic surface. The shear force generated at the wetted surface of the droplet plays in significant role on the adhesion force.