Session X39: Interfacial Phenomena II

Particle Capture on Oil-Coated Fibers

Oil-coated fibers are ubiquitous and useful for mist elimination, smog capture, and particle collection, to name a few. However, the interaction between impacting particles and oil-coated fibers has not been fully understood. In this study, we use two oil-coated parallel fibers to investigate particle capture dynamics. We identify that changing the amount of oil on the fibers and spacing between the fibers create various shapes of oil on the fibers from clam shell or barrel shaped droplets to columns connecting the fibers. On these oil-coated fibers, we explore the effects of particle size and impacting velocity, fiber flexibility, and fluid properties on particle capture dynamics. In particular, flexible fibers deform along the direction that particles impact, decreasing the relative velocity between the impacting particles and fibers, which improves particle capture efficiency compared with rigid fibers. Extending the fundamental understanding of the particle-fiber interaction, we demonstrate that microplastic particles are captured by oil-coated fibers in the air and water more efficiently compared to fibers without oil coating. We envision that the systematic understanding from this study could be applied to a larger scale particulate matter and microplastics collection systems.   

Design of a Liquid Impregnated Surface with a Stable Lubricant Layer in a Mixed Water/Oil Environment for Low Hydrate Adhesion

Clathrate hydrate is a naturally occurring icelike solid that forms in the water phase under suitable temperature and pressure conditions in the presence of one or more hydrophobic molecules. It also forms inside the oil and gas pipes, leading to higher pumping cost, flow blockage, and even catastrophic accidents. Engineered surfaces with low hydrate adhesion can provide an effective solution to this problem. Liquid impregnated surfaces (LIS) are examples of engineered surfaces that have already shown tremendous potential for reducing the nucleation and adhesion of solids. Here, we report the design and synthesis of liquid impregnated surfaces with extremely low hydrate adhesion under an oil and water mixed environment. The most challenging aspect of designing these surfaces was to stabilize a lubricant layer simultaneously under water and oil. A detailed methodology to make such lubricant-stable surfaces from a theoretical perspective was described and experimentally validated for lubricant stability. Experimental measurements on such surfaces showed extremely low hydrate accumulation and one order of magnitude or more reduction in hydrate adhesion force.   

Super aligned annular ring patterns of self-assembled nanomaterials

Cellulose nanocrystals (CNCs) offer great potential as a matrix for plasmonic metasurfaces composed of anisotropic gold nanorods (GNRs) due to their renewability, biodegradability, non-toxicity, and low cost. However, precise control over CNC positioning and orientation on substrates remains still challenging. In this talk, we presented a one-step fabrication method for a homogeneous quadrant CNC matrix film by integrating two interfacial phenomena: (i) spontaneous and rapid self-dewetting, and (ii) evaporation-induced self-assembly (EISA). By optimizing the composition of the CNC ink and determining the coating parameter, we achieved a balance between these two mechanisms, resulting in the alignment of CNCs in an annular shape and the production of coffee-ring-less CNC films. The hydrophilic surface of CNCs facilitated the continuous movement of the contact line without stick-slip motion during the drying process, enabling CNC films to exhibit a consistent annular ring pattern. Moreover, through the co-assembly of GNRs onto the CNC matrix, we created uniformly dried CNC-GNR films. This high level of uniformity and alignment improved their plasmonic performance. This work proposes a new physicochemical pathway for advancing the field of next-generation nanomaterials based on CNC control.   

Wetting Interactions of Anode and Electrolyte in Liquid Metal Batteries

In liquid metal batteries, there are three liquid layers (anode, electrolyte, and cathode) stably stratified by density in a metal container. It is important to the function of the battery that the anode maintains contact with the negative current collector, often composed of stainless steel. However, if the electrolyte has superior wetting it can cling to the collector, forcing the anode away. In this study, we characterized the wetting of both anode and electrolyte materials and the interactions between them by using the sessile drop method to measure their contact angles on stainless steel plate. Impurities in the materials and interactions between the layers have a significant effect on the wetting of the system. In order to prevent the displacement of the anode, we want to find a combination of materials that minimizes the anode contact angle and maximizes the electrolyte contact angle.   

Laboratory Measurements of Capillary-gravity Wave Scattering from Barriers with Contact Line Effects

Contact lines at a three-phase boundary (solid, liquid and air) play an essential role in the dynamics of the free surface of liquids in surface-tension-dominated fluids. While previous studies on the contact line effect have mainly focused on frequency and damping of standing wave modes in capillary dynamics, our study focuses on the contact line effect on capillary-gravity wave scattering from barriers. Models have predicted the contact line effects on capillary-gravity wave scattering from a barrier in ideal fluid configurations, but the lack of experimental data has hindered the progress. This research presents an experimental study that utilizes an acoustic approach to measure variations of the scattering with the barrier depth, barrier width, and surface wave frequency. Our study provides both evidence and quantitative measurements of the contact line effect on capillary-gravity wave scattering in realistic fluid configurations.   

Dynamic capillarity-driven texture morphing in flexible fins

Elastocapillarity, the capillary-induced deformation of slender materials, is a common phenomenon observed in our everyday activities like painting, washing hair, and seeing wet grass or leaves. In this study, we develop an unusual actuation system influenced by hydrodynamics, capillarity, and elasticity. We create an elastomeric fin placed between two parallel walls with different spacing gaps to break the symmetry. The submerged fins of the soft cell exhibit bimorphic textures upon emerging, dictated by the drainage rates governed by a syringe pump. Slow drainage results in a flexible fin to deform towards the narrow side due to a strong capillary pressure within the limited space (N-mode). Conversely, during rapid drainage (W-mode), the fin inclines towards the wider side, largely due to the suction pressure generated within the soft cell by drainage dynamics. Accounting for the viscous, capillary, and dynamic factors, we establish a theoretical model to offer a comprehensive regime map for the texture patterns of these flapping fins. Additionally, we provide a more complex polymorphism within the flapping fin array. We predict that this novel switchable texture system can lead to several practical uses, including reflective displays and multimodal signaling systems.   

Complex membrane formation using surfactant-regulated Marangoni instability at immiscible liquid-liquid interface

Interfacial polymerization (IP) is a method to fabricate uniform and defect-free polymer nanofilms for various academic and industrial applications, including sensors, electronics, batteries, catalysts, drug carriers, and separation membranes. The IP process occurs at an immiscible liquid-liquid interface through a type of step-growth polymerization. Therefore, understanding a mass transport mechanism is crucial to control the IP process. Specifically, surfactants have been introduced for the IP process, which can change the membrane’s characteristics, for instance the complexity of the interface shape and the filtration efficiency of polymer membranes. So far, various studies attempted to understand how nano-materials form at the interface, and several hypotheses have been introduced to explain the instability-triggering mechanism. However, the interfacial instability with surfactants is still unclear due to the lack of direct experimental observation. Thus, we decided to observe an interfacial flow field near the immiscible liquid-liquid interface. Here, the instability-driven mechanism underlying the surfactant-regulated interfacial formation of PA nanofilms was clarified by in situ visualization via particle image velocimetry (PIV). To focus on interfacial phenomena at the immiscible liquid-liquid interface, support-free interfacial polymerization (SFIP) was used to assemble a freestanding PA nanofilm at the interface between MPD (m-phenylenediamine) aqueous and TMC (trimesoyl chloride) organic solutions. A series of representative anionic (sodium dodecyl sulfate, SDS), cationic (dodecyl trimethyl ammonium bromide, DTAB), and neutral (Triton X-100, TX-100) surfactants by varying concentrations were added to the MPD solution to characterize the effect of different surfactants to the formation of PA nanofilms. Finally, we concluded that the critical role of Marangoni instability induced by the surfactants via various mechanisms in structurally regulating the nanofilms by observing the interfacial flow patterns.

  

Fast particle dispersion on air-water interface

Many powders rapidly spread radially outwards when introduced at an air-water interface. For instance, submillimeter-sized hollow glass spheres we use in experiments, exhibit an initial spreading speed of nearly 1 m/s and the spreading radius follows a self-similar behavior in time.

One hypothesis is, the particles are spread radially outwards by capillary waves generated at the instant when the particles impact the water surface. However, this spreading is significantly suppressed with hydrophobic and clean (of impurities) hydrophilic particles, an empirical fact inexplicable by the capillary wave hypothesis. Our findings suggest a surfactant-driven mechanism contributes to this explosive particle dispersion on air-water interface.   

Laminar liquid jet impinging on a vial creates dancing fluid sheets called water bells

We present the result of an experimental investigation of different fluid sheet structures formed during the impingement of a laminar liquid jet on the surface of the same liquid filled in a vial with a slightly larger diameter than the jet. The liquid while gushing out of the vial forms different shapes of the fluid sheet depending upon the input control parameters - the jet diameter and the jet velocity at the impact, the vial diameter, and the fluid properties. We present a detailed description of the water sheet structures observed and classify them into four regimes based on the governing non-dimensional parameters identified using Buckingham Pi theorem. The identified non-dimensional parameters are Jet to vial diameter ratio (X), Capillary number (Ca), Webber number (We), and Fruid number (Fr). In Regime I: pre-sheet regime, the formation of the liquid sheet structure is not observed; Regime II: puffing, is characterized by the intermittent formation and destruction of the transient upward rising water sheet. Regime III shows steady upward, inverted umbrella-like, sheet structures. Regime IV is distinguished by the formation of downward, umbrella-like, sheet structures, which are classically known as water bells in the literature.   

Vapor-powered boat

We present a novel self-propelling aquatic actuator inspired by Microvelia's movement, where the emission of a body fluid at the rear induces water-walking motions through the Marangoni force, rather than relying on mechanical forces like rowing or sliding. Previous soap- or cocktail-powered boats mimicked such mechanisms, but direct bulk mixing of liquid or solid fuel with water resulted in an instantaneous motion due to the rapid fuel consumption, and challenges in speed and direction control. To overcome these limitations, our study utilizes an alcohol vapor fuel to induce Marangoni force owing to the adsorption of alcohol vapor at the water-air interface, enabling sustainable movement on water surfaces with controllable speed and direction. We visualize these interfacial phenomena using a high-speed camera and Schlieren methods and construct a simple scaling model to rationalize the observed interfacial dynamics.   

Manipulation of Multiphase Fluid Using Photo-responsive Surfactants

Manipulating multiphase fluids is essential for applications such as thermal management in buildings and electronics, power generation, and desalination. Most previous works utilize passive fluid control through micro-/nanostructured surfaces, while active methods offer real-time tunability and multi-functionality, often relying on high voltage, strong magnetic fields, or high-temperature gradients.

 

Our work introduces dynamic control of droplets and bubbles in a multiphase fluid system using light-sensitive surfactants. These surfactants induce large interfacial tension changes, exerting a photo-Marangoni force on droplets/bubbles. By using light with lower intensity than in thermal-capillary actuation, we demonstrate fast, programmable droplet movement on liquid-infused surfaces, within microchannels, and on liquid substrates, and reveal results of bubble migration in gravity and microgravity conditions.

 

These results open exciting opportunities for the dynamic manipulation of multiphase fluid systems using photo-sensitive surfactants, benefiting areas including energy production, thermal management, and microfluidics.

  

Particle-laden capillary-driven flow in a vertical tube

Capillary tube flow has far reaching applications seen in automotive, HVAC, medical and low-gravity environments. The quantification of capillary rise in a vertical tube under varying conditions is important for these industries and continues to be explored. The focus of this research is on the inclusion of particles to capillary flow, more specifically investigating the arrangement of a particle-laden surface within the capillary tube. In medical and low-gravity environments it could be useful to move fluid carrying particulates, and understanding the dynamics of capillary-driven flow under these conditions is crucial to the development of technology and applications in these fields. Furthermore, this work provides fundamental information on mass transfer within a capillary-driven flow scenario. The research presented utilizes the Dryden Drop Tower at Portland State University to investigate the capillary rise in a vertical tube with a particle-laden surface. The drop tower allows access to a microgravity environment in which the length scales of capillary flow are substantially larger than would be in a 1-g laboratory environment. The rise velocity of the meniscus is measured for various different masses of particles at the surface within the tube.