Yanchu Liu

As a transitional flow pattern, slug–annular flow occurs over a wide range of operating conditions in micro-channels while its distribution in parallel micro-channels has not been well characterized. Herein, we conducted an experiment to study the phase distribution of slug–annular flow in parallel micro-channels. The test section consists of a header with a diameter of 0.48 mm and six branch channels with a diameter of 0.40 mm. Nitrogen and 0.03 wt% sodium dodecyl sulfate (SDS) solution were used as the test fluids. It was found that the phase distribution of the slug–annular flow was unstable and the duration of the varying process showed regularity with different inlet conditions. Increasing the liquid superficial velocity facilitated the liquid phase to flow into channels at the fore part of the header, while the channels at the rear part of the header were more supplied with liquid as the gas superficial velocity, volume fraction of gas, and volume flow rate increased. Furthermore, the results indicated that the channels located at the rear part of the header experienced a pronounced enhancement in the supply of both the liquid and gas phases, with the spacing between the branches increasing. A predictive correlation was formulated to ascertain the distribution of the liquid phase within slug–annular flow across parallel micro-channels.

The dynamic response of a micrometer-sized gallium–indium (Ga-In) jet in nitrogen subjected to intense Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser pulses with energy ranging from 0.35 to 5.0 mJ per pulse is investigated experimentally. The rapid deformation of the jet was visualized using timed-delayed stroboscopic shadowgraphy. The laser pulse disrupts the jet to form a gap, and the length of the gap grows according to a logarithm relationship with respect to the dimensionless time normalized a characteristic timescale τ, which is determined by the pulse energy Ep. The ablation impulse bends and flattens the jet into a thin curved film that resembles a wind-blown sail. The area of the sail increases with t6/5Ep13/15, where t is time. The sail eventually breaks up into fine mist. Additionally, we found that the laser-blast-induced initial bending velocity of the jet could be predicted using the semi-empirical laser-ablated propulsion model for an In-Sn droplet of tens of micrometers.

Depositing drops on a solid surface without entrapping bubbles is desirable for many spray coating and printing applications. Tian et al. [ J. Fluid Mech. 946, A21 (2022)] reported that an electric field can be applied to eliminate air bubble entrapments for neutral drops. Herein we provide a complete physical picture of the entire process of a drop impacting onto the solid surface under an external electric field. The electrohydrodynamic behavior during the drop impact is divided into three stages: the deformation of the drop in the electric field prior to contact, the initial contact of the drop with the substrate, and the rich postcontact phenomena including spreading, receding, jetting, and fragmentation. The results show that under the increasingly stronger electric fields, the modest drop oscillation transforms into a vertically stretched spindle. As the drop approaches the substrate, the electric stress at the south pole increases rapidly, which sharpens the bottom surface into a conical shape. The cone angle is determined by both the impact velocity and the electric field strength. After the contact, the surface electric stress tends to pull the drop upward, breaking up the drop, forming several jetting modes, and reducing the maximum spreading radius. The various drop deposition modes are summarized in a phase diagram, which sheds light on identifying appropriate electric fields for high-quality drop depositions without air bubble entrapments or jettings.

When a charge neutral drop impacts on a flat solid substrate, a small air bubble is always trapped underneath due to the lubrication pressure coming from the viscous stress in the squeezed air film. Herein we find experimentally and numerically that the process of the air entrapment and the initial contact state of the drop with the substrate can be profoundly altered via an external electric field. In an electric field, the induced electric stresses at the bottom of the drop increase drastically right before the drop contacts the substrate, which acts against the lubrication pressure, resulting in reduced initial contact radius and air bubble size. When the external electric field reaches a critical value, the electrical stress accelerates the flow near the bottom of the drop and generates a conical tip quickly instead of a dimple, resulting in a centre contact and eliminating the air bubble entrapment. Based on the dipole mirror charge model, we find the dimensionless strength of critical electric field scales with the square root of capillary number based on the air viscosity. This scaling law of the critical electric field for eliminating the air bubble entrapment is verified experimentally and numerically. This work may offer a new way to mitigate defects caused by air bubble entrapment for inkjet printing and droplet-based additive manufacturing.

The motion of millimeter gallium-indium (Ga-In) drops subject to intense Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser blasts in the air is investigated experimentally. The drop first experiences plasma emissions and then undergoes interfacial instabilities. The effective ablation pulse energy is quantified by the laser-induced shockwave propagation. The laser-blast-induced concave expansion and spanwise depression history is measured, and the data collapse on straight lines with proper rescaling of pulse energy and time. The propulsion speed of the drop is described by a semi-empirical model that considers the laser energy and fluence at the threshold of ablation. The data show that this propulsion speed scaling remains valid to the millimeter drop ablated by the pulsed laser with beam spot much smaller than the drop, although the original scaling was derived and verified for the indium-tin (In-Sn) droplet of tens of micrometers impacted by a laser pulse with the focal point larger than the droplet.

The motion of millimeter gallium-indium (Ga-In) drops subject to intense Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser blasts in the air is investigated experimentally. The drop first experiences plasma emissions and then undergoes interfacial instabilities. The effective ablation pulse energy is quantified by the laser-induced shockwave propagation. The laser-blast-induced concave expansion and spanwise depression history is measured, and the data collapse on straight lines with proper rescaling of pulse energy and time. The propulsion speed of the drop is described by a semi-empirical model that considers the laser energy and fluence at the threshold of ablation. The data show that this propulsion speed scaling remains valid to the millimeter drop ablated by the pulsed laser with beam spot much smaller than the drop, although the original scaling was derived and verified for the indium-tin (In-Sn) droplet of tens of micrometers impacted by a laser pulse with the focal point larger than the droplet.

The motion of millimeter gallium-indium (Ga-In) drops subject to intense Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser blasts in the air is investigated experimentally. The drop first experiences plasma emissions and then undergoes interfacial instabilities. The effective ablation pulse energy is quantified by the laser-induced shockwave propagation. The laser-blast-induced concave expansion and spanwise depression history is measured, and the data collapse on straight lines with proper rescaling of pulse energy and time. The propulsion speed of the drop is described by a semi-empirical model that considers the laser energy and fluence at the threshold of ablation. The data show that this propulsion speed scaling remains valid to the millimeter drop ablated by the pulsed laser with beam spot much smaller than the drop, although the original scaling was derived and verified for the indium-tin (In-Sn) droplet of tens of micrometers impacted by a laser pulse with the focal point larger than the droplet.