Laminar flow deformation of a droplet adhering to a wall in a channel

Deformation of Sessile Droplets under a Gentle Shear Airflow

Deformation of sessile droplets under shear flow is widespread in both nature and industry. Previous research focuses on the shedding process of sessile droplets under shear airflow, with insufficient attention paid to the droplet deformation before shedding. In this work, experimental studies on the deformation behaviors of sessile droplets under shear airflow are conducted to investigate the effects of airflow velocity and droplet volume on the tangential and normal droplet deformations. Scaling laws of the droplet deformations are established. The results show that the profile of sessile droplets changes under shear airflow with the topmost point exhibiting periodic oscillations in both tangential and normal directions. The oscillation period of the tangential deformation exceeds that of the normal deformation. The average tangential deformation of droplets increases with the increasing airflow velocity and droplet volume. The average normal deformation of droplets increases with the increasing airflow velocity and is influenced by the droplet volume at a higher airflow velocity. The contact angle on the windward side oscillates periodically, and its average value significantly decreases. The contact angle of droplets on the windward side decreases as the airflow velocity and droplet volume increase, while the contact angle on the leeward side remains almost unchanged. The average deformation of droplets in the tangential and normal directions is linearly related to the effective Weber number and the square of the effective Weber number. These findings could be used to predict the deformation of sessile droplets under shear airflows.

Refinements in the use of silicone oil as an intraocular tamponade

It is over 60 years since Paul Cibis et al. reported the experimental use of liquid silicone in the surgical management of retinal detachment. Initial experiences were complicated by significant side-effects associated with the impurities in the non-medical grade commercial silicone oils deployed at the time. These were substantially reduced (but not eliminated) by the adoption of refined high-viscosity medical grade silicone oils. Two of the major complications associated with silicone tamponade are (i) the variability of focus due to its movement and higher refractive index, and (ii) progressive emulsification, particularly with low viscosity oils. This article reviews recent and ongoing research on the causes of emulsification of intra-ocular silicone oil to understand the causes better and thereby reduce this risk, especially for those eyes where permanent tamponade is the only current option for retaining vision.

Fire ant rafts elongate under fluid flows

Fire ants survive flash floods by linking their bodies together to build waterproof rafts. Most studies of fire ant rafts consider static water conditions, but here, we consider the influence of flow. In particular, when floating on shallow water, the raft can run aground on vegetation, generating stresses in the raft as the water continues to flow around it. In this combined experimental and numerical study, we film the 10 h response of a fire ant raft caught on an anchor and subjected to water flows of 6 cm s^-1. In this situation, ant rafts elongate from circular to more streamlined shapes, doubling in aspect ratio before eventually contracting back into smaller circular shapes as they enter dormancy. Ants in upstream regions of the raft exhibit less exploration activity than those downstream, suggesting that ants migrate to areas of lower fluid stress. While the raft is rough, hydrophobic, and heterogeneous in height, we may gain some insight by performing both fluid-structure interaction and agent based simulations on smooth rafts. Elongation to the degree observed is associated with a 48% drag reduction. Moreover, a purely elastic raft does not elongate, but conversely increases its bluff body cross-sectional area. We conclude that ant raftsmust reconfigure to generate the elongated shape observed. This work may provide insights into designing intelligent robotic swarms that can adapt to fluid flows.

Improving heat and mass transfer rates through continuous drop-wise condensation

Drop-wise condensation (DWC) has been the focus of scientific research in vapor condensation technologies since the 20th century. Improvement of condensation rate in DWC is limited by the maximum droplet a condensation surface could sustain and the frequency of droplet shedding. Furthermore, The presence of non-condensable gases (NCG) reduces the condensation rate significantly. Here, we present continuous drop-wise condensation to overcome the need of hydrophobic surfaces while yet maintaining micron-sized droplets. By shifting focus from surface treatment to the force required to sweep off a droplet, we were able to utilize stagnation pressure of jet impingement to tune the shed droplet size. The results show that droplet size being shed can be tuned effectively by tuning the jet parameters. our experimental observations showed that the effect of NCG is greatly alleviated by utilizing this technique. An improvement by multiple folds in mass transfer compactness factor compared to state-of-the-art dehumidification technology was possible.

Continuous Movement Mechanism of Oil Droplets Adhered on Surfaces with Different Wettability in the Flow Field

In this aticle, the continuous movement patterns and characteristic parameters of oil droplets on surfaces with different wettability immersed in a laminar flow field were observed, and the change rules of the geometric parameters of oil droplets under different experimental conditions were obtained. Then, the factors influencing the continuous moving behaviors of the oil droplets were analyzed, and the continuous movement velocity of the oil droplets under different experimental conditions was demonstrated. On this basis, the change law of the continuous movement velocity of oil droplets with the flow velocity was discussed. In addition, the coupling effect of the oil drops' height, surface properties, and water flow velocity on the continuous movement of oil droplets was studied, and the critical conditions for the continuous movement were obtained. According to the critical conditions, the mathematical model which described the law of continuous motion of the oil droplets adhered on surfaces with different wettability in the laminar flow field was established. The quantitative relationships among the average continuous moving velocity of oil droplets, physical properties, geometric parameters, water flow velocity, and surface wettability were obtained, which defined the necessary conditions for the uniform and accelerated movement of oil droplets, providing an important basis for choices of suitable surface wettability and flow field conditions in practical engineering applications.

Modelling of immiscible liquid-liquid systems by Smoothed Particle Hydrodynamics

Immiscible fluid systems are ubiquitous in industry, medicine and nature. Understanding the phase morphologies and intraphase fluid motion is often desirable in many of these situations; for example, this will aid improved design of microfluidic platforms for the production of medicinal formulations. In this paper, we detail a Smoothed Particle Hydrodynamics (SPH) approach that facilitates this understanding. The approach includes surface tension and enforces incompressibility. The approach also allows the consideration of an arbitrary number of immiscible phases of differing viscosities and densities. The nature of the phase morphologies can be arbitrary and change in time, including break-up (which is illustrated) and coalescence. The use of different fluid constitutive models, including non-Newtonian models, is also possible. The validity of the model is demonstrated by applying it to a range of model problems with known solutions, including the Young-Laplace problem, confined droplet deformation under a linear shear field, and a droplet falling under gravity through another quiescent liquid. Results are also presented to illustrate how the SPH model can be used to elucidate the behaviour of immiscible liquid systems.

Predicting the wetting dynamics of a two-liquid system

We propose a new theoretical model of dynamic wetting for systems comprising two immiscible liquids, in which one liquid displaces another from the surface of a solid. Such systems are important in many industrial processes and the natural world. The new model is an extension of the molecular-kinetic theory of wetting and offers a way to predict the dynamics of a two-liquid system from the individual wetting dynamics of its parent liquids. We also present the results of large-scale molecular dynamics simulations for one- and two-liquid systems and show them to be in good agreement with the new model. Finally, we show that the new model is consistent with the limited data currently available from experiment.

Displacement of liquid droplets on a surface by a shearing air flow

The motion of droplets on surfaces is crucial to the performance of a wide range of processes; this study examines the initiation of droplet motion through a shearing mechanism generated here by a controlled air flow. Systematic experiments are carried out for a range of fluids and well defined surfaces. A model is postulated that balances surface tension forces at the contact line and the drag force due to the air motion. Experiments reveal that the critical velocity at which droplet motion is initiated depends on the contact angle and the droplet size. Visualizations highlight three modes of motion: (I) the droplet retains a footprint similar to that at the point of motion; (II) a tail exists at the rear of the droplet; (III) a trail remains behind the droplet (that can shed smaller droplets). The predictions of droplet initiation velocity are good for type I motion, in accordance with the assumptions inherent within the model. This model confirms the dominant physics associated with the initiation of droplet motion and provides a useful predictive expression.