Numerical Simulation of the Influence of Different Surface Morphologies on Molten Pool Flow under Moving Heat Source

In order to study the influence of different initial topography on the molten pool flow under a moving heat source, the finite element analysis method was used to establish a two-dimensional transient model of laser polishing to simulate the evolution of the surface topography of the material during laser polishing. In the simulation process, a moving laser beam was used as the heat source, and the free surface of the actual material was profiled through a three-dimensional profiler. A very similar simulation model surface was constructed, coupled with the flow field and temperature field in the laser polishing process, and the capillary force was considered comprehensively. Combined with thermocapillary force. The results show that under the combined action of capillary force and thermocapillary force, the surface of the polished material has a peak-filling effect, which makes the surface of the material achieve a good polishing effect. The initial shape will affect the polishing effect, the greater the curvature, the faster the flow rate of the molten pool. In molten pools with large spatial curvatures, capillary forces dominate. Keywords: Laser polishing; molten pool; surface topography; numerical analysis; capillary force; thermocapillary force.

A Transformative Gold Patterning through Selective Laser Refining of Cyanide

Gold is an essential noble metal for electronics, and its application area is increasing continuously through the introduction of gold nanoparticle ink that enables rapid prototyping and direct writing of gold electrodes on versatile substrates at a low temperature. However, the synthesis of gold nanoparticles has certain limitations involving high cost, long synthesis time, large waste of material, and frequent use of chemicals. In this study, we suggest simultaneous laser refining of gold cyanide and selective fabrication of gold electrodes directly on the substrate without a separate synthesis step. Gold cyanide is commonly the first product of gold from the primitive ore, and the gold can be extracted directly from the rapid photothermal decomposition of gold cyanide by the laser. It was confirmed that laser-induced thermocapillary force plays an important role in creating the continuous gold patterns by aligning the refined gold. The resultant gold electrodes exhibited a low resistivity analogous to the conventional direct writing method using nanoparticles, and the facile repair process of a damaged electrode was demonstrated as the proof-of-concept. The proposed transformative approach for gold patterning, distinguished from the previous top-down and bottom-up approaches, has the potential to replace the well-known techniques and provide a new branch of electrode manufacturing scheme.

Numerical Investigation On Bubble Growth and Merger in Microchannel Flow Boiling with Self-rewetting Fluid

The application of two-phase flow in microchannel needs further research to achieve a more stable and highly-performed heat sink. Utilizing self-rewetting fluid is one of the promising ways to minimize the dryout area, thus increasing the heat transfer coefficient and critical heat flux (CHF). To investigate the heat transfer performance of self-rewetting fluid in microchannel flow boiling, a numerical investigation is carried out in this study utilizing the VOF method, phase-change model and continuum surface force (CSF) model with surface tension versus temperature. Athree-dimensional numerical investigation of bubble growth and merger is carried out with water and 0.2%wt heptanol solution. The single bubble growing cases, two x-direction/y-direction bubbles merging cases and three bubbles merging cases are conducted. Since the bubbles never detach the heated walls, the dryout area and regions nearby the contact line with thin liquid film dominated the heat transfer process during the bubbles' growth and merger. The self-rewetting fluid is able to minimize the local dryout area and achieve the larger thin liquid film area around the contact line due to the Marangoni effect and thermocapillary force, thus result in higher wall heat flux when compared to water. The two x-direction bubbles merging case performed best for heat transfer in the microchannel, in which self-rewetting fluid achieves heat transfer enhancement for over 50 percent compared with water.

Influence of Lateral Restraint on Thermocapillary Migration of Wetting Droplets

The present study aims at studying the characteristics of thermocapillary migration with varying levels of lateral restraints. A temperature gradient is created by heating and cooling either side of the substrate. When a droplet is placed near hot side it spreads as thin film and migrates towards the cold side. The advancing end assumes the shape of a parabolic rim while the receding end stays as a thin film. It is observed that the droplet decelerates to attain a steady state velocity and undergoes slight acceleration near the cold end of the substrate. The observed velocity trend follows the temperature gradient on the substrate. The velocity increases with the droplet volume and substrate temperature gradient. The liquid viscosity is observed to have a diminishing effect on migration velocity. The effect of lateral spread confinement is studied by performing experimental trails on substrates with different widths. It is found that reducing the substrate width increases the migration velocity due to increased footprint resulting in larger thermocapillary force. The results observed in the present study highlights the importance of thermocapillary flows in many academic and industrial applications.

Simulation of Single Bubble Evaporation in a Microchannel in Zero Gravity With Thermocapillary Effect

This paper presents fundamental research on the hydrodynamics and heat transfer surrounding a single elongated bubble during flow boiling in a circular microchannel. A continuum surface force (CSF) model based on the volume of fluid (VOF) method is combined with the thermocapillary force to explore the effects of thermocapillarity for flow boiling in microchannels. To validate the self-defined codes, a two-phase thermocapillary-driven flow and a Taylor bubble growing in a capillary tube are studied. Results of both test cases show good convergence and agreement with data from the earlier literature. The bubble motion and the local heat transfer coefficient (HTC) on the heated wall with respect to time are discussed. It is found that for large Marangoni number (case 3), variation of surface tension has affected the bubble shape and temperature profile. The thermocapillary effect induces convection in a thin liquid film region, which augments the HTCs at specified positions. The numerical investigation also shows that the average HTC increased by 6.7% in case 3 when compared with case 1. Thus, it is very important to study further the effects of themocapillarity and the Marangoni effect on bubble growth in microchannels.

Effect of thermocapillarity and variable thermal conductivity on the heat transfer analysis of a non-Newtonian liquid thin film over a stretching surface in the presence of thermal radiation and heat source/sink

An analysis illustrating the flow of an Ostwald-de-Waele liquid film on an unsteady stretching sheet under the influence of thermocapillary force, magnetic field and viscous dissipation is carried out. In this study, thermal conductivity is assumed to be a function of fluid temperature. Numerical solutions for the partial differential equations governing the flow are obtained by employing the elegant Runge-Kutta-Fehlberg method for certain representative values of controlling parameters, such as thermocapillarity number, magnetic field parameter, etc. Film thickness is calculated for various values of flow parameters. Film thickness of shear thinning fluids is found to be smaller than that of a Newtonian fluid and a converse trend holds true for shear thickening fluids. Thicker films are noticed for increasing values of thermocapillarity number. In the presence of thermocapillary force, an initial decrease in the velocity of a shear thinning fluid occurs before fluid velocity experiences a significant increase towards the free surface. Stronger magnetic field strengths are seen to increase the free surface velocity. Themocapillary force on temperature in a shear thinning fluid is more prominent.

Influence of Ambient Airflow on Free Surface Deformation and Flow Pattern Inside Liquid Bridge With Large Prandtl Number Fluid (Pr > 100) Under Gravity

The influence of airflow shear on the free surface deformation and the flow structure for large Prandtl number fluid (Pr = 111.67) has been analyzed numerically as the parallel airflow shear is induced into the surrounding of liquid bridge from the lower disk or the upper disk. Contrasted with former studies, an improved level set method is adopted to track any tiny deformation of free surface, where the area compensation is carried out to compensate the nonconservation of mass. Present results indicate that the airflow shear can excite flow cells in the isothermal liquid bridge. The airflow shear induced from the upper disk impulses the convex region of free interface as the airflow shear intensity is increased, which may exceed the breaking limit of liquid bridge. The free surface is transformed from the “S”-shape into the “M”-shape as the airflow shear is induced from the lower disk. For the nonisothermal liquid bridge, the flow cell is dominated by the thermocapillary convection at the hot corner if the airflow shear comes from the hot disk, and another reversed flow cell near the cold disk appears. While the shape of free surface depends on the competition between the thermocapillary force and the shear force when the airflow is induced from the cold disk.

Role of Capillary and Thermocapillary Forces in Laser Polishing of Metals

As one of emerging novel surface treatment techniques, laser polishing offers a cost-effective and efficient solution to reduce surface roughness of precision components at micro-/mesoscale. Although it has been applied for industrial and biomedical purposes, the underlying mechanism has not been fully revealed. This paper presents a study to understand the basic fundamentals of continuous wave fiber laser polishing of Ti6Al4V samples. A two-dimensional numerical model that coupled heat transfer and fluid flow is developed to illustrate the molten flow behavior. The roles of capillary and thermocapillary flow in the process of laser polishing are investigated to assist the understanding of the contributions of surface tension (capillary force) and Marangoni effect (thermocapillary force) in the polishing process. Capillary force dominates the molten pool at the initial stage of melting, while thermocapillary force becomes predominant when the molten pool fully develops.