A multiphysics model for charged liquid droplet breakup in electric fields

Additive manufacturing is driving major innovations in many areas such as biomedical engineering. Recent advances have enabled 3D printing of biocompatible materials and cells into complex 3D functional living tissues and organs using bioink. Inkjet-based bioprinting fabricates the tissue and organ constructs by ejecting droplets onto a substrate. Compared with microextrusion-based and laser-assisted bioprinting, it is very difficult to predict and control the droplet formation process (e.g., droplet velocity and size). To address this issue, this paper presents a new data-driven approach to predict droplet velocity and size in the inkjet-based bioprinting process. An imaging system was used to monitor the droplet formation process. To investigate the effects of excitation voltage, dwell time, and rise time on droplet velocity and droplet size, a full factorial design of experiments was conducted. Two predictive models were developed to predict droplet velocity and droplet size using random forests. The accuracy of the two predictive models was evaluated using the relative error. Experimental results have shown that the predictive models are capable of predicting droplet velocity and size with sufficient accuracy.

Predictions of the droplet size and velocity distributions in sprays under isothermal conditions are reported. The calculations are based on the maximum entropy formalism, complying with the conservation laws of liquid mass, momentum and energy. This theoretical approach considers only the macroscopic quantities about the atomization processes, without resorting to the details of the liquid breakup processes such as the onset and growth of instabilities. The derived joint droplet size and velocity distribution function depends on the Weber number as well as the liquid mass, momentum and energy source terms.These parameters represent the conditions under which the atomization occurs. The droplet velocity distributions are truncated Gaussian distributions for any specific sizes. The nondimensional Sauter mean diameter decreases slightly with the Weber number and then approaches an asymptotic constant. The calculated values of D21/D30 are very close to unity which agrees with the experimental observations. The computations also show that the atomization efficiency is very low; less than 2.6 percent.

Droplet trajectories of wave-impact sea spray on a marine vessel

The characteristics of twin-fluid atomization operating in the annular flow regime were studied experimentally under various gas-to-liquid ratios (GLRs) and injection pressures. The macroscopic morphology of the spray was obtained by shadowgraph, while the droplet size and velocity were measured using a phase-Doppler particle analyzer technique. It was found that the spray cone angle increases almost linearly with the GLR, and the axial distance required for droplet coalescence to outweigh the breakup decreases with increasing GLR. The Sauter mean diameter (SMD) first decreases and then increases along the axial direction due to the competition between turbulent breakup and droplet coalescence. The droplet size follows a lognormal distribution; the droplet velocity distribution is closer to a lognormal distribution under large GLRs, while it follows normal distribution with GLR = 3%. Regarding the radial distribution, low GLRs (3% and 5%) lead to a bimodal spatial velocity distribution, while for large GLRs, the droplet velocity decreases monotonically toward the far field. The spray tends to become more stable with increasing GLR and injection pressure Pinj, whereas the SMD increases with increasing Pinj. The underlying atomization mechanism in a twin-fluid injector in the annular flow state can be regarded as the disintegration of the initial liquid sheet by longitudinal Kelvin–Helmholtz instability followed by transverse Rayleigh–Taylor instability, which yields a direct proportionality of the droplet size to the initial liquid sheet thickness ΔL. Subsequently, for high Pinj, the gas core shrinks and ΔL increases, which results in an increased SMD but enhanced atomization efficiency ΔL/SMD.

Measured values of fuel droplet velocity, size and flux are presented for a vaporizer based on a T-shaped duct with upstream atomization by a single axial jet and by six radial jets. They were obtained for a practical range of kerosene and air flowrates and inlet air temperatures with the vaporizer in free air and in a sector of an annular combustor with combustion. Phase Doppler velocimetry was used to measure droplet velocity and size distributions and was complemented by photographic visualization of the flames within the combustor. The results obtained outside the combustor, and without combustion, showed that the Sauter mean diameter of the droplets ranged from 20 to 60 μm and the liquid-fuel flux from 0.2 to 30 per cent of the total fuel as the inlet air temperature was increased from that of ground-idle to that of full power. The droplet size and liquid-fuel flux also diminished with an increase in air flowrate, and an arrangement of six radial jets resulted in better atomization than an axial arrangement. The corresponding fluxes with combustion were in the range between 0.1 and 8 per cent as a consequence of heat transfer from combusting gases to the vaporizer tubes. Experience with the vaporizer operating within the combustor at fuel flowrates and inlet air temperatures representative of take-off showed that the vaporizer performance could deteriorate rapidly due to the formation of carbon deposits, particularly in the region where the flow impinged on the cross tube. The deposits led to reduced heat transfer and vaporization with a consequently larger proportion of larger droplets and a tendency for the region of intense combustion to move downstream.

Techniques for measurement of droplet size and velocity distributions in agricultural sprays

The properties of the flow from fire water nozzles, like droplet size and velocity distribution within the spray, are known to influence the fire suppression efficiency. To analyze the flow properties, the water spray is recorded with the use of a high-speed camera and laser light. Typically, each image of the water spray may contain tens of droplets, yielding a huge number of possible droplet paths between adjacent frames, i.e. � droplets in two subsequent frames generates � 2 possible droplet paths with �! possible configurations using brute-force approach. In this paper, we propose an optimization method based on the Hungarian algorithm to calculate the droplet paths. Using this framework, each droplet path is optimized with respect to droplet position, droplet size and droplet velocity.

A free energy model is used to describe the droplet formation and break-up process in a T-junction bio-microchannel. Droplets are created as a result of interaction of two immiscible liquids. Different stages for the droplet formation process are analyzed which are: a) growing in the x and y directions, b) growing in the x direction and c) detachment process. The effects of capillary number and flow rate ratio on the droplet formation stages are also studied. The influences of Capillary number, flow rate ratio, viscosity ratio and geometrical parameters on droplet break up, droplet size and detachment time are systematically studied. By increasing the flow rate ratio; the duration of droplet formation and the length of the x-growth stage are decreased for small capillary numbers. For larger capillary numbers; the droplet penetrates toward the downstream; therefore, the length of the x-growth stage is increased. The start of detachment process in the microchannel is also reported, which is related to narrowing of the neck of the liquid film. The results show that the detachment time is increased by decreasing the Capillary number. For Ca>0.02, the detachment time is independent of the flow rate ratios. Moreover; the effects of viscosity ratios on detachment time are not significant in comparison to the effects of capillary number. For Ca<0.04, the size of the droplet is independent of the viscosity ratio, but after the critical Capillary number (i.e., Ca=0.04), the size of the droplet is varied by the viscosity ratio. The time between two consecutive drops is also decreased by increasing the Capillary number. Moreover, this time is decreased by increasing the flow rate ratio until Ca=0.04. After this Capillary number, the flow rate ratios have no significant effect on the time between two consecutive droplets. An exhaustive validation study is performed including (a) the Laplace equation in the stationary droplet; (b) a contact angle test; (c) Taylor deformation test in shear flow and (d) comparison of the droplet length as a function of flow rate ratio between the present work and other studies.

Experimental study of liquid aluminum droplet breakup characteristics based on a Drop-on-demand (DOD) magnetohydrodynamic actuation

An experimental investigation on the electrospray characteristics in a meso-scale system at different modes

An electrospray with a coflowing nebulizing air is characterized for three different electrospray modes: spindle, cone jet, and unstable. An electrospray with dimensions used in a mass spectrometer with 150-μm ID nozzle was characterized. The droplet size and velocity distributions were measured using a Phase Doppler Particle Analysis (PDPA) with and without the coflowing air. The significance of the coflowing air on the final spray droplet size and velocity distribution is found to depend on the electrospray mode. Generally, the coflowing air increases both the droplet size and velocity distributions, and the increase is more significant in the spindle mode than the cone jet mode.

Numerical and experimental investigation of induced flow and droplet–droplet interactions in a liquid spray

Droplet formation and breakup dynamics in microfluidic flow-focusing devices: From dripping to jetting

Microdroplet formation in microfluidic systems plays a pivotal role in chemical engineering, biomedicine, and energy applications. Precise control over the droplet size and formation dynamics of microdroplets is essential for optimizing performance in these fields. This work explores a hybrid control strategy that combines an active electric field with passive rib structures to regulate the droplet formation in a ribbed T-junction microchannel under an electric field. Numerical simulations based on the phase-field method are employed to analyze the effects of the electric capillary number and rib height on the droplet formation mechanism. The results reveal that increasing induces three distinct flow regimes of the dispersed phase: unpinning, partially pinning, and fully pinning regimes. This transition from an unpinning to a pinning regime increases the contact area between the wall and dispersed phase, restricts the flow of the continuous phase, and induces the shear stress of the wall, leading to a reduction in droplet size with the enhanced . Furthermore, an increase in rib height enhances the shear stress of the continuous phase above the rib, causing a progressive shift from a fully pinning to an unpinning regime, which results in a linear decrease in droplet size. A new empirical correlation is proposed to predict droplet size as a function of rib height and two-phase flow rate ratio : .

Biofabrication offers a great potential for the fabrication of three-dimensional living tissues and organs by precisely layer-by-layer placing various tissue spheroids as anatomically designed. Inkjet printing of living cell-laden bioink is one of the most promising technologies enabling biofabrication, and the bioink printability must be carefully examined for it to be a viable biofabrication technology. In this study, the cell-laden bioink droplet formation process has been studied in terms of the breakup time, droplet size and velocity, and satellite formation using a time-resolved imaging approach. The bioink has been prepared using fibroblasts and sodium alginate with four different cell concentrations: without cells, 1 × 10(6), 5 × 10(6), and 1 × 10(7) cells/mL to appreciate the effect of cell concentration on the droplet formation process. Furthermore, the bioink droplet formation process is compared with that during the inkjetting of a comparable polystyrene microbead-laden suspension under the identical operating conditions to understand the effect of particle physical properties on the droplet formation process. It is found that (1) as the cell concentration of bioink increases, the droplet size and velocity decrease, the formation of satellite droplets is suppressed, and the breakup time increases, and (2) compared to the hard bead-laden suspension, the bioink tends to have a less ejected fluid volume, lower droplet velocity, and longer breakup time.