High-speed imaging in fluids

High-speed imaging is in popular demand for a broad range of experiments in fluids. It allows for a detailed visualization of the event under study by acquiring a series of image frames captured at high temporal and spatial resolution. This review covers high-speed imaging basics, by defining criteria for high-speed imaging experiments in fluids and to give rule-of-thumbs for a series of cases. It also considers stroboscopic imaging, triggering and illumination, and scaling issues. It provides guidelines for testing and calibration. Ultra high-speed imaging at frame rates exceeding 1 million frames per second is reviewed, and the combination of conventional experiments in fluids techniques with high-speed imaging techniques are discussed. The review is concluded with a high-speed imaging chart, which summarizes criteria for temporal scale and spatial scale and which facilitates the selection of a high-speed imaging system for the applicatio

Drop coalescence with liquid/liquid interfaces in the presence of surfactants

The dissertation describes the experimental investigations on the effect of surfactants on drop coalescence with liquid/liquid interfaces. Different coalescence events are carried out in two unique-designed vessels at different dimensions. Optical methods including high-speed shadowgraphy and Particle Image Velocimetry (PIV) are applied to detect the drop surface behaviour and the drop inner dynamics. Planar Laser Induced Fluorescence (PLIF) is also utilized to detect the spatio-temporal distribution of surfactants on the coalescing drop surface. A novel flow channel is developed to investigate the surfing drops at moving liquid/liquid interfaces at last. Initially, the total coalescence of drops with the liquid/liquid interfaces, both packed with surfactants at concentrations up to the Critical Micelle Concentration (CMC), are studied. It is found that the increase of the surfactant concentration promotes the deformation of the interface before the film that separates the drop from the interface drains and ruptures. After the film rupture, two counter-rotating vortices appear inside the drop, which move faster at low surfactant concentrations. The intensities of the two counter-rotating vortices significantly decrease at increasing surfactant concentration. In the second part of the dissertation, the experimental results on the partial coalescence of drops are presented. The coalescence map based on the dimensionless numbers Oh=µ_d/(ρ_mσD)^1/2 and Bo = (ΔρgD^2)/σ is plotted to distinguish the partial coalescence region, which is found to reduce in the presence of surfactants. The size ratios of the daughter to the mother drop are measured and plotted against the Bo and Oh as well. In the gravity dominant regime, the surfactants have a negligible effect on the drop ratio, while in the inertia-capillary regime the drop size ratio decreases with increasing surfactant concentration. The velocity fields inside the partial coalescing drop are acquired. In the surfactant-free system, it is found that the inward motion of the fluids at the upper part of the drop favours the generation of a liquid cylinder in the early stages. The pressure gradient created by the downward stream at the bottom of the liquid cylinder is believed to cause the pinch-off of the cylinder and the formation of the secondary droplet. The surfactants tend to make the coalescence non-symmetric resulting in total coalescence. The spatiotemporal distributions of surfactants on the merging interfaces are presented in the following part. It is found that when a drop rests on an interface, the surfactants that have been adsorbed on the interfaces are swept outwards by the draining liquid film between the drop and the flat interface and reach a peak value at about 0.75R away from the center, where R is the horizontal drop radius. After the film ruptures, the surfactant concentration at the tip of the retreating meniscus continues to increase. Once the film has retracted to the drop sides, the concentration of the surfactants peaks at the meniscus at the bottom of the drop. The variation of the surfactant concentration along the merging interfaces in the later stages is presented as well. In the end, the delayed coalescence of drops with moving liquid-liquid interfaces is experimentally investigated. Drops are released onto the moving interfaces at velocities from 0 cm/s up to 3.4 cm/s. Drop coalescence is found to be largely delayed at increasing interface speed, which is attributed to the lubrication pressure developed in the draining film. Numerical simulations are conducted for a half-pendent drop levitating on a moving liquid-liquid interface. The results indicate that the minimum pressure appears at the front bottom of the moving drop, where most of the film ruptures are observed to take place. Also, the pressure in the film is calculated based on the local curvature of the drop surface and the tangential velocities on the drop surface and on the interface in the film region. It is found that the lubrication pressure increases with the interface speed. The PLIF technique is used to measure the drop shape and the film thickness between the drop and the interface. A dimple structure in the film is more likely to form at lower interface speeds, while the film tends to be flat when the interfaces move with high speed. During coalescence of drops with moving interfaces, in some cases when films are thicker, the liquid in the film cannot completely drain out but form drops-on-string resulting in the oil entrainment in the aqueous phase

Deformation and breakup of single drop in laminar and transitional jet flows

The authors gratefully acknowledge the financial support from the National Key R&D Program of China (2017YFB0306701), National Natural Science Foundation of China (No.21676007)，the Fundamental Research Funds for the Central Universities (XK1802-1), and Scientific Research and Technology Development Projects of China National Petroleum Corporation (No. 2016B-2605).Peer reviewedPostprin

A standing Leidenfrost drop with Sufi-whirling

The mobility of Leidenfrost drop has been exploited for the manipulation of drop motions. In the classical model, the Leidenfrost drop was levitated by a vapor cushion, in the absence of touch to the surface. Here we report a standing Leidenfrost state on a heated hydrophobic surface where drop stands on the surface with partial adhesion and further self-rotates like Sufi-whirling. To elucidate this new phenomenon, we imaged the evolution of the partial adhesion, the inner circulation, and the ellipsoidal rotation of the drop. The stable partial adhesion is accompanied by thermal and mechanical equilibrium, and further drives the development of the drop rotation.Comment: 16 pages, 4 figure

Flows inside polymer microfluidic droplets: Role of elasticity

The role of elasticity on the flow topology inside viscoelastic (Boger) droplets moving in a rectangular microchannel is examined experimentally by means of micro Particle Image Velocimetry (μPIV). Polyacrylamide (PAAM) water -glycerol solutions of different concentrations are employed to vary droplet elasticity. Varying the Wi number alters the flow topology inside the Boger microdroplets, progressively reducing the number of vortical structures observed until their complete disappearance. The flow structure resembles that of Newtonian inelastic droplets for low Wi and elasticities. However, when the Wi and elastic number increase above one-i.e. elastic effects become more important- a new flow structure is observed at the front of the droplets characterized by two recirculating regions either side of the droplet centreline. Spatiotemporal maps show that the flow in this new regime fluctuates periodically around the centreline indicating the onset of an elastic instability. This flow transition is attributed to the well documented coupled effects of polymer stretching and curved streamlines and resultant hoop stresses. The findings highlight the importance elasticity can have on the nature of microdroplet flows of complex fluids and the potential of tuning elasticity to engineer flow structures for given microfluidic applications

Experimental and Numerical Modeling of Fluid Flow

This Special Issue provides an overview of the applied experimental and numerical flow, models, which are used to investigate fluid flow in complex situations. The investigated problems are related to fundamental processes or new applications. As demonstrated, the field of the application of experimental and numerical flow models is constantly expanding

CHARACTERIZATION OF THE INITIAL SPRAY FROM A JET IN CROSSFLOW

An experimental study on the initial spray from a liquid jet in air crossflow was conducted using Shadowgraphy and Particle Image Velocimetry (PIV) techniques. Momentum ratio and gas Weber number were varied to study their effects on the column trajectory, spray trajectory, breakup locations and spray characteristics after column breakup. Correlations for column trajectory, spray trajectory, breakup locations in terms of momentum ratio and gas Weber number were obtained using linear regression of the experimental data. Two breakup modes were recognized in the test (Column breakup and Bag breakup), a breakup mode regime map was provided including effects of momentum ratio and gas Weber number. Drop characteristics in the spray were also investigated