Bubble rising and interaction in ternary fluid flow: a phase field study.

Bubble-droplet interaction is essential in the gas-flotation technique employed in wastewater treatment. However, due to the limitations of experimental methods, the details of the fluid flow involved have not been fully understood. Therefore, a phase field model for a three-phase flow was developed to study the rise of a single bubble and bubble-droplet interactions. The fluid-fluid interfaces are tracked by the Cahn-Hilliard equation, which is coupled with the Navier-Stokes equations with an equivalent volumetric force substituted for interfacial tensions. The model was discretized using an explicit finite difference method on a half staggered grid, and the pressure velocity coupling was tackled using the projection method. The in-house code was written in Fortran and run with the help of OpenMP, a shared memory parallelism. The model was validated against experiments with gratifying agreement achieved. Bubble-droplet interaction was simulated in two distinct situations: the first features a gas bubble crossing the interface between two other phases, and the second features a gas bubble chasing from behind an oil droplet in a surrounding fluid of the third phase.

A Conservative Level Set Approach to Non-Spherical Drop Impact in Three Dimensions.

A recently developed conservative level set model, coupled with the Navier-Stokes equations, was invoked to simulate non-spherical droplet impact in three dimensions. The advection term in the conservative level set model was tackled using the traditional central difference scheme on a half-staggered grid. The pressure velocity coupling was decoupled using the projection method. The inhouse code was written in Fortran and was run with the aid of the shared memory parallelism, OpenMP. Before conducting extensive simulations, the model was tested on meshes of varied resolutions and validated against experimental works, with satisfyingly qualitative and quantitative agreement obtained. The model was then employed to predict the impact and splashing dynamics of non-spherical droplets, with the focus on the effect of the aspect ratio. An empirical correlation of the maximum spread factor was proposed. Besides, the number of satellite droplets when splashing occurs was in reasonable agreement with a theoretical model.

Improved Resolution and Fidelity of Droplet-Based Bioprinting by Upward Ejection.

Bioprinting has emerged as a powerful biofabrication technology with widespread applications in biomedical fields because of its superiority in high-throughput, high-precision, 3D structure fabrication. For bioprinting, two of the most important parameters are the printing precision (i.e., droplets resolution) and structural fidelity (i.e., conformity of the printed objects to the design). The major factors that hinder resolution and fidelity are gravity and impact force between printed droplets and substrate. However, existing solutions to these two issues, including decreasing droplet volume and introducing sacrificial materials, cause other problems, such as complexity or poor biocompatibility. Here, we reported a variant 3D bioprinting technique, termed as upward bioprinting, in which the nozzle of bioprinter is overturned and the ejection direction is opposite to gravitational force. Employing this technique, we fabricated discrete droplets, continuous lines, and 3D multilayer constructs using alginate and gelatin methacrylate (GelMA). The characterizations show that the upward bioprinting could improve the resolution and also fidelity as compared with the conventional downward bioprinting. Meanwhile, this method enables cell printing without affecting the viability. In addition, this method can be easily implemented without upgrading any hardware. Such an upward bioprinting technique could be an alternative to scale down microtissues and to fabricate 3D complex bioconstructs. We envision that the upward bioprinting, as a general method, could be extended to other bioprinting processes or applied to 3D bioprinting in outer space.