Droplet Mobility Research Articles

Movement of oil droplets against salt concentration gradients in thin capillaries

Mobilization of residual oil droplets is the key process for enhanced oil recovery. Visualization of the droplet movement at a pore level provides insights on the underlying physical mechanisms. We couple a microfluidic droplet generator and a thin glass capillary to study the movement of oil droplets under salinity gradients with visualization of individual droplet movements. The driving forces that affect the movement of the droplets are discussed. We demonstrate experimentally that oil droplets in micro-confined channels can be mobilized and move against pressure under the concentration gradients of dissolved salts. The gradient-driven movement can be strong enough to drive a droplet through a narrow constriction in the middle of the capillary channel. The droplet movement can be understood by combining a Marangoni stress due to surfactant redistribution, electrostatic interaction and diffusiophoresis. This suggests that the abrupt change of salinity may be one of the physical mechanisms of smart waterflooding.

Superior anti-icing performance of a silicone-based slippery gel coating with a self-replenishing lubricant layer under airflow

Icing phenomena lead to energy consumption, mechanical failures, and potential human casualties. These issues often occur under dynamic airflow conditions. While superhydrophobic (SHPo) surfaces effectively delay ice formation under airflow, they cannot completely prevent it without external energy. This study proposes a long-chain entangled polydimethylsiloxane (LEP) gel coating infused with a lubricant, as an excellent anti-icing surface. The LEP gel has a self-regenerative, low-viscosity oil layer, and it shows remarkable droplet mobility, a very low contact angle hysteresis (CAH), and near-zero ice adhesion. X-ray microimaging shows that condensed droplets on the LEP gel move dynamically, while a bare silicone coating exhibits static behavior. This dynamic droplet behavior significantly delays droplet freezing and frost onset. The LEP gel’s anti-icing performance is assessed in a subsonic wind tunnel. Unlike control surfaces on which frost forms immediately, droplets condense on the gel and then slide off, reducing the frost-coverage rate. Notably, at a wind speed of 5.5 m/s, no frost is formed on the gel. The LEP gel’s superior anti-icing performance is attributed to synergistic effects of the lubricant layer and extremely low CAH. These results show that lubricant-infused slippery gel coatings could prevent icing in airflow environments.

Enhancing Directional Droplet Transport via Surface Charge Gradient: Insights from Molecular Dynamics Simulations.

The phenomenon of spontaneous droplet transport has a wide range of implications in water collection, microfluidic manipulation, oil-water separation, and various other fields. Achieving efficient and controllable spontaneous droplet transport is therefore of paramount importance. This study investigates the potential of surface charge manipulation to enhance spontaneous droplet transport through comprehensive molecular dynamics simulations. Our findings reveal that the surface charge of the substrate significantly influences its wettability, reducing the contact angle of the droplet and increasing both the contact area and interaction energy. Moreover, we introduce a novel approach to enhance droplet mobility by creating a surface charge gradient on the substrate. By introducing bands with varying charges along a specific direction of the substrate, the droplet experiences a force directed toward regions of increasing charge, thereby facilitating its movement. Importantly, the driving mechanism of droplet motion is well explained by combining classical electrowetting theory with the analysis of the droplet's advancing and receding contact angles, which demonstrates that a more pronounced surface charge gradient generates greater force and enhances droplet mobility. These findings offer valuable insights into the design of microfluidic systems and related applications based on electrowetting.

Drop Retention and Departure in Adiabatic Shear Flow on Structured Superhydrophobic Surfaces.

Drops are retained or held on surfaces due to a retention force exerted on the drop by the surface. This retention force is a function of the surface tension of the liquid, drop geometry, and the contact angle between the drop and the surface. When external or body forces exceed the retention force, the drop begins to move. This work explores the conditions for which drop departure occurs on structured superhydrophobic surfaces in the presence of an applied shear flow. Drop departure is explored for five microstructured superhydrophobic surfaces, one nanostructured carbon nanotube surface and one smooth hydrophobic surface. Surface solid fractions range from 0.05 to 1.00, and measured static contact angles range from 121° to 161°. Droplet volumes of 5, 10, 20, 30, 40, and 50 μL are tested on each surface. For each experiment, increasing air velocity is applied to a droplet placed on a surface until the droplet departs. High-speed imaging is used to track droplet base length, height, cross-section area (as viewed from the side) and advancing/receding contact angles. Measurements of drop advancing and receding contact angles are reported at the point of departure, with increasing contact angle hysteresis observed prior to departure. Contact angle hysteresis is observed to be a good indicator of droplet mobility. Measurements of the average air velocity over the height of the droplet are determined at the point of departure for all conditions. The measured air velocity shows strong dependence on the surface solid fraction, and the required shear flow velocity decreases as the surface solid fraction decreases. This is most pronounced at very low solid fractions. A coefficient of drag for the departing drops in shear flow is calculated and is shown to decrease with increasing Reynolds number.

Mechanism chaotic movement of Leidenfrost droplets

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

PH drives electron density fluctuations that enhance electric field-induced liquid flow

Liquid flow along a charged interface is commonly described by classical continuum theory, which represents the electric double layer by uniformly distributed point charges. The electrophoretic mobility of hydrophobic nanodroplets in water doubles in magnitude when the pH is varied from neutral to mildly basic (pH 7 → 11). Classical continuum theory predicts that this increase in mobility is due to an increased surface charge. Here, by combining all-optical measurements of surface charge and molecular structure, as well as electronic structure calculations, we show that surface charge and molecular structure at the nanodroplet surface are identical at neutral and mildly basic pH. We propose that the force that propels the droplets originates from two factors: Negative charge on the droplet surface due to charge transfer from and within water, and anisotropic gradients in the fluctuating polarization induced by the electric field. Both charge density fluctuations couple with the external electric field, and lead to droplet flow. Replacing chloride by hydroxide doubles both the charge conductivity via the Grotthuss mechanism, and the droplet mobility. This general mechanism deeply impacts a plethora of processes in biology, chemistry, and nanotechnology and provides an explanation of how pH influences hydrodynamic phenomena and the limitations of classical continuum theory currently used to rationalize these effects.

Insights into nanostructured silica particle formation from sodium silicate using spray drying: An experimental and theoretical study

Spray drying is a widely used operational procedure utilized in diverse sectors. The current knowledge of droplet mobility processesinside the drying chamber is not fully comprehensive due to the complex nature of droplet evaporation issues. The present study used a comprehensive methodology that integrated numerical models and actual testing to investigate the drying process of droplets in the context of spray drying. The precursor material utilized in this investigation was colloidal silica. The influence of external hydrodynamic variables, such as drying temperature (473, 673, and 873 K) and carrier gas flow rate (2, 4, 6 L/min), was thoroughly investigated in connection to the dynamics of droplets and the subsequent creation of silica particles. Distinct alterations in the size and shape of the silica particles were observed due to variations in drying temperature and carrier gas flow rate. The validity of this investigation was established by the generation of two discrete particle morphologies, namely spherical particles and doughnut-shaped particles. Spherical particles were produced under reduced carrier gas flow rates of 2 L/min and lower drying chamber temperatures of 473 K. In the quantitative evaluation, the computational fluid dynamics (CFD) computations properly established the evaporation rate constant (K) and the temperature difference (ΔT) inside the droplets. Furthermore, qualitative calculations provide insight into the complex dynamics of droplet motion inside the spray chamber. The results of this study have the potential to open up new possibilities for regulating nanoparticle creation via the use of the spray drying process.

Excellent hydrophobicity and high mobility of condensate droplets on hierarchical nanostructured surfaces: Insights from MD simulations

Comprehending the fundamental physical mechanisms behind condensation and the formation of suspended droplets on hierarchical structured surfaces is of paramount importance in the design of anti-frosting surfaces. In this study, we characterize the nucleation, growth and dynamic wetting behaviors of condensate droplets on hierarchical nanostructured surfaces by molecular dynamics simulation, and reveal that hierarchical structures facilitate the rapid formation and high mobility of suspended droplet during condensation. The mechanism underlying the formation of suspended droplets on hierarchical surfaces is unveiled, which can be attributed to the spontaneous dewetting transition induced by confined growth of individual droplets. Additionally, the impact of solid fraction and secondary structure distribution on condensation and dynamic wetting is systematically discussed. The results indicate that an increase in solid fraction or the number of secondary structures enhances the stability of suspended droplets and improves the droplet mobility. In particular, the presence of bottom secondary structures on sidewalls plays a crucial role in the formation of suspended droplets on hierarchical surface. It is expected that the above findings could provide valuable guidance for optimizing the design of functionalized surfaces with anti-frosting and self-cleaning performance.

Microscopic behavior of nano-water droplets on a silica glass surface

Recent advancements in computational science and interfacial measurements have sparked interest in microscopic water droplets and their diverse behaviors. A previous study using nonlinear spectroscopy revealed the heterogeneous wetting phenomenon of silica glass in response to humidity. Building on this premise, we employed high-resolution atomic force microscopy to investigate the wetting dynamics of silica glass surfaces at various humidity levels. Our observations revealed the spontaneous formation of nano-water droplets at a relative humidity of 50%. In contrast to the conventional model, which predicts the spreading of nanodroplets to form a uniform water film, our findings demonstrate the coexistence of nano-water droplets and the liquid film. Moreover, the mobility of the nano-water droplets suggests their potential in inducing the transport of adsorbates on solid surfaces. These results may contribute to the catalytic function of solid materials.

Investigating the influence of pore-widening time control in electrochemical anodization for the formation of hybrid titanium nanostructures with enhanced superhydrophobicity for improved corrosion resistance efficiency

Titanium (Ti) alloys possess remarkable strength, high-temperature stability, and processability, making them indispensable in biomaterials, aircraft engines, fuselage manufacturing, and ship propulsion shafts. However, when exposed to potential differences or minor defects in contact with other alloys, the thin natural oxide film on their surface is vulnerable to damage, leading to accelerated corrosion or diminished stability. This study investigated the morphology of the final oxide film formed with varying pore-widening (PW) process time (10–60 min with 10-minute intervals) after the anodization of Ti-Grade 4 material. The oxide films formed from the anodization and the PW process for 10 to 40 min exhibited a porous structure, while a hybrid pillar and pore structure were observed after a PW process of 50 min. As the PW time increases, the pore diameter expands, forming hybrid pillar and pore nanostructures, and superhydrophilicity is observed. After FDTS (1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane) coating, the phenomenon elucidates the transition from hydrophobicity to superhydrophobicity and the enhancement of water droplet mobility, with the lowest contact angle hysteresis measured at PW of 60 min and demonstrating improved corrosion resistance, achieving a corrosion inhibition efficiency of over 99% compared to the untreated titanium alloy surface. Notably, the structure encompassing pillar and pore features exhibits exceptional suitability for applications on material surfaces and interfacial phenomena, such as cell adhesion. Furthermore, surfaces characterized by excellent superhydrophobicity because of coating can find multifunctional applications encompassing anti-corrosion, anti-fouling, anti-icing, and self-cleaning capabilities.

Multifunctional integrated pattern for enhancing fog harvesting water unidirectional transport in a heterogeneous pattern

Solid surfaces with improved wettability as well as geometric structures can enhance capture and droplet removal, thereby improving fog harvesting. We fabricated Al wires by combining superhydrophilic (SHL), superhydrophobic (SHB), and oil-infused SHB (SHBO) surfaces into a pattern whose fog-harvesting efficiency could be measured. The SHL-SHBO-SHL pattern showed the highest promise of water droplet capture and mobility on a solid surface with 42% efficiency compared to the 34% efficiency of Bare. In order to identify the optimal efficiency features, two boundary conditions (boundary I: from SHL to SHBO and boundary II: from SHBO to SHL) were introduced, and the impact of the hydrophilic area was examined. Boundary I boosts capture efficiency whereas boundary II increases drain efficiency. Understanding the forces operating at the wettability gradient surface, as well as incorporating the area ratio of SHL and SHBO via wettability combinations, are key to designing effective fog harvesting systems.

Solvent-free synthesis of self-regenerative superhydrophobic film from silicone waste with drag-reduction and selective oil absorption behaviour

In the present work, we devised an ecological solution for recycling silicone waste into functionalized films possessing superhydrophobicity with low drag and oil adsorption characteristics. We used two ecological approaches to transform silicone waste into nanoparticles with inherently distinct surface characteristics. These spheroidal silica nanoparticles, with an average size of 150 nm, showed hydrophilic and hydrophobic characteristics. The detailed characterization confirmed hydrophobic silica being grafted with –CH3 oligomers compared to hydrophilic counterparts. These silica nanoparticles were used to fabricate flexible superhydrophobic films through a solvent-free approach. Films synthesized using hydrophilic particles showed better de-wetting characteristics with advancing angle > 150°, low hysteresis (<5°) and sliding angle (<5°) at an optimum volumetric concentration. High de-wetting stability and low adhesion of ∼ 20 μN with water droplet were noticed for these films due to homogenous particle distribution, leading to significant protrusions and favourable line tension. High van der Waals interaction energy aided better mixing of hydrophilic particles in the matrix than the hydrophobic counterpart. This also resulted in high mechanical strength ∼ 7 MPa (matrix strength ∼ 3 MPa) and better robustness under different harsh conditions, sustaining more than ten months of weathering and one month of immersion. Films showed real-time self-regeneration capability, retaining high de-wetting due to surface chemistry and morphology rejuvenation. The film also displayed extremely low drag, promoting water droplet mobility at ∼ 0.5 m/s wind velocity and oil–water separation capability. The fluorine-free flexible superhydrophobic films synthesized using silicone waste exhibited multi-dimensional characteristics for wide-range applications.

Vibration‐Enabled Mobility of Liquid Metal

AbstractDirected liquid metal (gallium‐based) manipulation and actuation are paramount for copious applications, including soft robotics, soft electronics, and targeted drug delivery. Although there are several strategies available to achieve mobility of liquid metals in a “wet” environment. Strategies to achieve and improve mobility of liquid metal droplets and puddles in a “dry” environment are scarce and rely on metallophobic surface design or liquid metal marbles. Here, high mobility of Galinstan is elucidated by combining metallophobic surface design and vertical vibrations. Vibration frequencies between 20 and 30 Hz are conducive to droplet movement and threshold inclination angles of 0.5° to 1° are observed upon actuation by these vibrations. The method itself is applicable for a wide range of droplet sizes (30 and 2000 µL) and very robust. The droplet movement typically comprises of periodic receding and advancing of the droplet and commences via a rolling mechanism rather than a gliding mechanism. Finally, it is shown that small (0.5 mm height) obstacles can be traversed by this method, indicating that it can be used in concert with other strategies, such as surface structuring strategies, which open up pathways for mobility and controlled actuation of liquid metal droplets in air.

Nanorough Is Not Slippery Enough: Implications on Shedding and Heat Transfer.

Lowering droplet-surface interactions via the implementation of lubricant-infused surfaces (LISs) has received important attention in the past years. LISs offer enhanced droplet mobility with low sliding angles and the recently reported slippery Wenzel state, among others, empowered by the presence of the lubricant infused in between the structures, which eventually minimizes the direct interactions between liquid droplets and LISs. Current strategies to increase heat transfer during condensation phase-change relay on minimizing the thickness of the coating as well as enhancing condensate shedding. While further surface structuring may impose an additional heat transfer resistance, the presence of micro-structures eventually reduces the effective condensate-surface intimate interactions with the consequently decreased adhesion and enhanced shedding performance, which is investigated in this work. This is demonstrated by macroscopic and optical microscopy condensation experimental observations paying special attention at the liquid-lubricant and liquid-solid binary interactions at the droplet-LIS interface, which is further supported by a revisited force balance at the droplet triple contact line. Moreover, the occurrence of a condensation-coalescence-shedding regime is quantified for the first time with droplet growth rates one and two orders of magnitude greater than during condensation-coalescence and direct condensation regimes, respectively. Findings presented here are of great importance for the effective design and implementation of LISs via surface structure endowing accurate droplet mobility and control for applications such as anti-icing, self-cleaning, water harvesting, and/or liquid repellent surfaces as well as for condensation heat transfer.

Microvesicle-Embedded Solid-liquid Composite Coating for the Tribological Behavior Regulation and Long-Acting Lubrication.

Friction interfaces in liquid-embedded composite lubrication coatings commonly comprise a combination of discontinuous fluid films and rough solid contact surfaces, which together ensure easy shearing and a prolonged wear life. However, achieving high efficacy in mixed lubrication poses a challenge due to the conflicting nature of enhanced migration freedom for the liquid lubricant and increased mechanical strength of the solid matrix. Recent efforts have focused on incorporating reinforcing fillers to develop multicomponent, multiphase composites that can address this paradox. Here, we describe a modified attapulgite (APT) with strong biphasic wettability via the oil decompressive osmosis treatment on APT nanocontainers grafted with long nonpolar alkyl chains. This modified APT enables control over the size, distribution, and mobility of lubricant droplets by constructing a Pickering emulsion and toughens the solid-phase matrix through dispersion strengthening. Additionally, the introduction of APT induces the formation of a solid tribofilm during friction, which possesses a higher oil adsorption capacity, as verified through first-principles calculations based on density functional theory (DFT). Consequently, the fluid films can be replenished by the fracture of nanocontainers and adsorption from the bulk phase; further comprehensive and effective regulation of the friction interface leads to low friction and wear.

Electrophoresis of hydrophobic and polarizable liquid droplets in hydrogel medium

The emulsion droplet-hydrogel composite is a new class of biomaterial which finds potential application in several fields, including chemical, biological, food and pharmaceutical industries, as well as to controlled release drug in targeted location, etc. Besides, the electric field response of polarizable emulsion droplets quenched in hydrogel medium offers several Lab-on-a-Chip applications. Note that the gel electrophoresis of such droplets is advantageous compared to free-flow electrophoresis. Given the importance, a general theory of electrophoresis applicable for any hydrophobic and polarizable liquid droplets quenched in hydrogel medium is developed. Such a theory is also applicable for electric field response of emulsion droplets quenched in a hydrogel medium. The mathematical model is based on conservation principles of mass and momentum considering the hydrogel skeleton as soft polymeric material saturated with electrolytic solution. Within the Debye-Hückel electrostatic framework, we have derived a closed form analytical expression for electrophoretic mobility of hydrophobic and polarizable liquid droplets embedded in hydrogel skeleton. The derived general expression is further approximated with negligible error and the corresponding approximate form is found to be very convenient and can be easily programmed on a personal computer. Additionally we further deduce several closed form analytical expressions derived under various limits. Thus, the newly derived analytical results may be useful to the experimentalists to analyze and interpret their observations.

Remediation of DNAPL-contaminated heterogeneous aquifers using colloidal biliquid aphron: Multiscale experiments and pore-scale simulations

The heterogeneity of the aquifer seriously affects the remediation efficiency of dense non-aqueous phase liquid (DNAPL) due to the low contact efficiency for remediation reagents and DNAPL at macro scale and the restricted moving of DNAPL droplets by capillary forces in pore at micro scale. In this work, colloidal biliquid aphron (CBLA) as a remediation reagent was proposed, it has the rheological properties of shear thinning and the ultra-low interfacial tension between DNAPL. This work investigated the mechanisms and efficiencies of remediation DNAPL-contaminated heterogeneous aquifers using CBLA by sandbox and micromodel scale. The results indicated that CBLA could improve the mass transfer efficiency by both increasing the sweeping and extraction efficiency. The remediation efficiency of tetrachloroethylene (PCE) in aquifers with different permeability was above 99 % in sandbox, with the highest effluent concentration of 1.89 × 105 mg/L. Furthermore, at micromodel scale, CBLA could raise the remediation efficiency by improving the mobility and emulsification of PCE droplets. There was no PCE residue in the fine pore throats, and the remediation efficiency was 99.49 % in micromodel experiment and simulation. This work provided a promising remediation reagent for removing DNAPL from heterogeneous aquifers and laid a theoretical foundation for its practical application.

Effect of roughness on droplet motion in a capillary channel: A numerical study

This study presents droplet dynamics in a rough capillary channel. Prior studies investigating the effect of roughness on fluid flow have mainly considered a continuous phase whose behavior is different from a discontinuous phase, i.e., an oil slug. To explore the dynamic behavior of droplet motion across a rough channel, a direct numerical simulation of in a three-dimensional channel is performed. Three models have been considered: model A had a rough surface only on the bottom walls, model B on both the bottom and top walls, and model C on all walls. The results show that in contrast with common observations, roughness promotes droplet mobility in comparison with smooth walls. The presence of roughness results to an additional energy required to move the droplet, and the degree of confinement increases with the roughness; thus, the difficult of mobilization increases with the increase in roughness. Different roughness parameter effects have been investigated. The results have shown that the critical pressure increases with the increase in the pillar's height and decreases with the pillars spacing. The offset leads to a decrease in flow resistance for larger contact angles. We noted also that it is more difficult to mobilize a discontinuous phase in a neutral-wet surface condition. Furthermore, discontinuous pillars in the lateral direction led to much higher resistance. Through our comprehensive numerical study, we provide valuable insights into the impact of roughness in capillary channels. These findings can be used as guidelines for designing droplet flow on complex and rough surfaces, such as microfluidic devices, and hold significant relevance in the optimization of droplet control strategies in enhanced oil recovery methods.

Droplet slipperiness despite surface heterogeneity at molecular scale

Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10° to 110° yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets.

Interaction of Blood and Bacteria with Slippery Hydrophilic Surfaces

AbstractSlippery surfaces (i.e., surfaces that display high liquid droplet mobility) are receiving significant attention due to their biofluidic applications. Non‐textured, all‐solid, slippery hydrophilic (SLIC) surfaces are an emerging class of rare and counter‐intuitive surfaces. In this work, the interactions of blood and bacteria with SLIC surfaces are investigated. The SLIC surfaces demonstrate significantly lower platelet and leukocyte adhesion (≈97.2% decrease in surface coverage), and correspondingly low platelet activation, as well as significantly lower bacterial adhesion (≈99.7% decrease in surface coverage of live Escherichia Coli and ≈99.6% decrease in surface coverage of live Staphylococcus Aureus) and proliferation compared to untreated silicon substrates, indicating their potential for practical biomedical applications. The study envisions that the SLIC surfaces will pave the path to improved biomedical devices with favorable blood and bacteria interactions.