Peng Sheng Wei

This study investigates fluid flow, heat transfer, and energy generation induced by surface plasmon resonance in an array of nanorods (NRs) on a metal surface irradiated by a laser in the transverse magnetic mode. Surface plasmons are electromagnetic waves that propagate along the interface between two media, resulting in both distributed and localized heat input on the surface. The heating, melting, and solidification behaviors of NRs or nanoparticles are crucial for understanding additive manufacturing, plasma processing, and various nanotechnology applications. Simulations performed with COMSOL software, which solve the transport and electromagnetic equations, reveal the dependence of surface plasmon or energy absorption on the incident angle, the width and frequency of the laser, and the number of NRs on the metal surface. Oscillations of surface plasmons lead to initial melting at distinct locations, with the peak surface temperature approximately coinciding with the peaks of energy generation. A secondary energy-generation peak, caused by the penetration of a strong electric field intensity, appears near the triple-phase line of the first NR. The molten pools due to thermocapillary flow subsequently expand and coalesce, enclosing voids and forming pores that transition from deformed to cylindrical shapes between the NRs.

Solute segregation around an isolated pore in a solid, resulting from bubble entrapment under impact flow during upward-advancing solidification, is numerically predicted. In addition to influencing the microstructure of structural materials, porosity in functional materials has applications in biology, tissue engineering, food preservation and addressing global warming through its impact on sea ice. This study extends a previous model by incorporating impact flow at the top surface and examines solute fields around a fully entrapped pore in the solid. The impact velocity during solidification can be regarded as the relative velocity between the liquid and the solidification front. By solving the transport equations for mass, momentum, energy and concentration using COMSOL software, it is shown that solute distribution in the solid surrounding the pore and its shape can be controlled by adjusting transport-related parameters, including transport-related factors, such as impact velocity, gravity acceleration, ambient pressure, and solid thermal conductivity, as well as metallurgical factors like Henry's law constant, mole fraction of the solute gas at the top free surface, and partition coefficient. These parameters enable preemptive management of solute segregation and the shape of an entrapped pore in solid. The predicted contact angle during solidification closely aligns with Abel's equation, experimentally confirming previously.

This study explores sufficient conditions based on transport parameters to effectively manipulate the growth and shape of lotus pores during solidification. Specifically, spatial variations in mass transfer coefficients and solute transport parameters are considered. Solute transport is the key factor responsible for the evolution of lotus pores. The work investigates the influence of these conditions on the shape development of lotus pores, which find applications in diverse fields such as energy absorption, sound absorption, shock and vibration damping, heat sinks, filters and tissue and bone implants. The unsteady gas pressure within the pores, responsible for bubble entrapment, is influenced by gas, capillary and liquid pressures, as well as Sieverts’ or Henry's laws, and solute transfer at gas–liquid interfaces. Solute transport into the pore is determined by liquid convection as well as convection-affected segregation at the advancing liquid–solid interface. The study utilizes the MATLAB code to solve the resulting first-order time-derivative ordinary differential equations. It shows that the lotus pores are susceptible to entrapment as the mass transfer coefficient at a contact angle of 90° increases, while the solute transport parameters at the initial time and a contact angle of 90° decrease. The lotus pore length increases and decreases, respectively, with increases in the mass transfer coefficients as well as the solute transport parameter at the initial time and a contact angle of 90°. Predicted lotus pore shapes align closely with algebraic results, validated by experimental data from a prior study.

This study scales the roughness, parallel to the scanning direction, on a solid surface solidified from thermocapillary surfaces under various working conditions as well as the gas inflow introduced from above. This roughness is commonly observed in autogenous and non-autogenous welding processes, additive manufacturing, and polishing techniques, leading to reduced yield, increased fatigue and fracture strength, and heightened stress concentration in the solidified area. The roughness of the solidified surface subjected to gas inflow from above is quantitatively scaled by applying the Young–Laplace equation, accounting for surface deformation due to differences in impact pressure across the free surface and dynamic pressure in the shear layer beneath the free surface. Surface velocity is scaled by balancing viscous stress with thermocapillary force and inertial force with viscous stress in the shear layer, while also considering mass conservation during melting and solidification. Results indicate that concave depth in the central region and bulge height at the edge increase as the impact pressure difference across the free surface, Weber number, Marangoni number, and Prandtl number increase, while the Peclet number decreases. The widths of the central and edge regions, which are independent of inflow velocity from above, increase with dimensionless beam power, solid-to-liquid thermal conductivity ratio, and other factors. COMSOL Multiphysics version 6.0 is used to compute surface deformation roughness influenced by unsteady two-dimensional fluid flow and heat transfer. The predicted roughness amplitude and pitch align well with scaling analysis data.

In this study, the development of solute concentration and velocity fields of a pair of entrapping or coalescing bubbles during downward solidification is provided. The gas-induced pores in the metal not only deteriorates the properties of the processed workpiece by causing stress concentration and defects within the material, but pore formation in sea ice also plays an important role in global warming. Using COMSOL Multiphysics version 5.2, the unsteady, two-dimensional transport equations of mass, momentum, energy, and concentration are solved. The results show that bubble coalescence is facilitated by decreasing solid thermal conductivity and interpore spacing. Unlike the symmetric distribution of concentration observed with a low Henry's law constant and liquid solute diffusivity, an asymmetric distribution occurs, with high and low concentration gradients near the leading and rear edges of each bubble, respectively, due to the liquid velocity from the upstream direction. An outward flow in the opposite direction occurs near the triple-phase line, resulting in an inflection region in the iso-concentration field. The thickness of the concentration boundary layer surrounding the pores also decreases with decreasing Henry's law constant and liquid solute diffusivity, as well as with increasing ambient pressure, gravitational acceleration, solid thermal conductivity, and surface tension. The predicted contact angle during solidification aligns well with Abel's equation. Solute segregation associated with the formation of multiple pores can be controlled.