Chun-Yu Zhang

The Allen-Cahn phase-field (ACPF) model is widely used for simulating melting and icing processes in turbulent flows, owing to its ability to handle complex geometries and multiphase systems. However, the original ACPF model cannot strictly enforce the condition that the temperature at the ice front equals the melting point when the ice front is curved. This condition is a physical requirement essential for macroscopic flows at millimeter or larger scales. The numerical temperature discrepancy leads to spurious phase change at curved fronts. To accurately simulate the evolution of the ice front, we propose an improved ACPF model. Specifically, a curvature counter term is introduced into the Allen-Cahn equation to suppress curvature-induced artificial phase change, and an effective melting point is employed in combination to ensure that the temperature at the ice front remains at the melting point. To enhance computational efficiency, the model is implemented within two open-source computational frameworks, i.e., AFiD and Basilisk. The proposed method is validated through a series of numerical tests, including ice-water coexistence, freezing and melting of a circular ice shape, melting of an icicle, ice growth under forced convection, and phase change in thermally driven turbulence. Results show good agreement with analytical solutions, previous numerical data, and experimental measurements. Finally, a large-scale three-dimensional simulation of a melting iceberg is presented to demonstrate the computational performance and scalability of the proposed approach.

Although various physical tweezers and external fields based on optical, magnetic, electrical, and acoustic interactions have been developed for noncontact droplet manipulation, existing techniques are limited by the use of responsive additives in droplets/substrates or changes in the droplet’s physical and chemical properties during operation. Herein, we report a wind tweezer (WT) that uses a controlled airflow field (wind force) to remotely and programmatically manipulate droplets in a highly precise and flexible manner. A comprehensive equation is developed to characterize the wind force acting on a droplet within the WT system, taking into account variables such as wind speed, WT height, pipe diameter, droplet size, and droplet’s deviation distance. The versatile WT can manipulate droplets with variable volumes (100 nL to 3 mL), chemical compositions, and high velocities (up to 160 mm/s), as well as droplet arrays on various substrates and Leidenfrost droplets on superheated smooth surfaces. Vertical droplet manipulation is also allowed. The WT has been successfully used in various exciting applications, such as circuit welding/repair, cell transport in bioactive liquids, and improving the detection performance of surface-enhanced Raman scattering (SERS).

The forced breakup of liquid jets in ambient gas surroundings is studied systematically through numerical simulations and theoretical analyses, with particular emphasis on characterising the response modes of jet breakup across wide ranges of perturbation frequency and amplitude. Simulations reveal that the breakup of liquid jet can be effectively synchronised with external actuation within a frequency range encompassing the natural breakup frequency, thereby enabling the generation of highly uniform droplets. As the perturbation frequency exceeds an upper critical value, the external perturbation cannot dominate the jet breakup, while below a lower critical frequency, the jet breaks up with multiple droplets generated within one period. A high perturbation amplitude can result in liquid accumulation, leading to the formation of a pancake-shaped jet configuration. Through spectrum analyses, the development of jet interface perturbations under different response modes is elucidated, revealing the competition between the natural frequency and the external frequency. A linear instability analysis of a liquid jet is performed, which successfully predicts the synchronised frequency range by comparing the breakup time between the free liquid jet and the actuated jet, along with the variation tendencies of jet breakup length with varying perturbation frequency, amplitude and jet velocity. Quantitative numerical results demonstrate that in the case of multiple droplet generation under low perturbation frequency, the rear droplet maintains a higher velocity than its leading counterpart and the emergence of a high-pressure zone at the leading edge of a droplet train facilitates the droplet coalescence. Furthermore, the study introduces an innovative approach by superimposing periodic pulses onto the sinusoidal perturbation waveform, enabling active modulation of multiple droplet merging dynamics. This fundamental study is intended to offer valuable guidance for the on-demand generation of droplets in various industrial applications.

Oscillating sessile droplets on a plane, crucial in industrial applications like inkjet printing, spray cooling, and antifogging, present a long-standing challenge in determining viscous damping. Existing solutions are limited to first-order asymptotic approximations of hemispherical droplets, accounting only for viscous dissipation near the wall. Considering further the bulk dissipation that is usually neglected due to its higher-order effect, we obtain a two-term approximation of decay rates for pinned sessile droplets with different contact angles. We demonstrate that the two-term approximation effectively resolves discrepancies between predicted and observed dampings for viscosities in the range of practical interest, supported by experiments and simulations. The two-term approximation also explains why hydrophobic droplets possess much lower damping than hydrophilic ones, and offers a template for damping prediction formulas with potential viscosity measurement applications.