In this letter, the process of printing and laser curing of nanoparticle solutions is presented. A liquid solvent is employed as the carrier of gold nanoparticles (the material of interest in this study) possessing a low melting temperature compared to that of bulk gold. Using a specifically designed printing system, the gold nanoparticle solution is deposited on a substrate and cured with laser radiation. In this manner, the potential of writing gold structures on temperature sensitive substrates is demonstrated. The interaction between the laser radiation and nanoparticles drives the solvent evaporation and controls the quality of the microstructures printing process. The latter is also affected by thermocapillary flow at the free surface, developing during the curing process. An optical method for estimating the curing times is also developed and discussed.

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This article presents a predominantly numerical investigation of the transient transport phenomena occurring during the pileup ͑deposition one upon another͒ of molten, picoliter-size liquid metal droplets relevant to a host of novel micromanufacturing processes. The investigated phenomena last fractions of a millisecond in severely deforming domains of typical size of a small fraction of a millimeter. The prevailing physical mechanisms of the pileup process ͑occurring simultaneously͒ are identified and quantified numerically. These are the fluid mechanics of the bulk liquid, capillarity effects at the liquid-solid interface, heat transfer, solidification, and thermal contact resistance effects at all interfaces. In terms of values of the Reynolds, Weber, and Stefan number the following ranges are covered: Reϭ281-453, Weϭ2.39-5.99, and Steϭ0.187-0.895. This corresponds to molten solder droplets impinging at velocities ranging between 1.12 and 1.74 m/s having an average diameter of Ϸ78 m. The initial substrate temperature ranges between 25 and 150°C. The initial droplet temperature is 210°C. The numerical model presented is based on a Lagrangian formulation of the Navier-Stokes equations accounting for surface tension, thermal contact resistance, solidification, and a Navier slip condition at the dynamic contact line. Results of simulations are presented showing the effect of thermal contact resistance and slip at the dynamic contact line on the transients and the outcome of a pileup. Comparisons of the simulated pileup with experimental visualizations are shown, demonstrating good agreement in cases where inertia dominates over capillary effects. For decreasing Stefan number ͑i.e., higher substrate temperatures͒ an increasing importance of wetting is observed. For these cases the limitations of the employed popular boundary condition at the dynamic contact line is demonstrated and the need for experimental data ͑currently nonexistent in the literature͒ that would yield an improved condition at the contact line accounting for the temperature dependence of wetting phenomena is underpinned.

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This paper focuses on the effect that surface tension (Marangoni phenomenon) and viscosity dependence on temperature has on the spreading, transient behavior and final post-solidification shape of a molten Sn63Pb solder droplet deposited on a flat substrate. A Lagrangian finite element formulation of the complete axisymmetric Navier-Stokes equations is utilized for the description of the droplet behavior. Linear temperature dependence for the surface tension and an exponential dependence for the viscosity are assumed. The initial droplet temperature is varied in 50 K steps from 200°C to 500°C, whereas the substrate temperature is kept constant at 25°C. This varies the initial Reynolds number Re0 from 360 to 716 and the Marangoni number Ma from −9 to −49. The initial Weber number We0 and initial Prandtl number Pr0 are for all cases O(1) and O10−2, respectively. The impact velocity and the droplet diameter remain unchanged in all cases examined at 1.5 m/s and 80 microns. A major finding of the work is that, contrary to intuition, the Marangoni effect decreased droplet spreading monotonically. Due to the Marangoni effect, the mechanism that arrested spreading is the surface tension and not the beginning of freezing. Droplet receding during recoiling was aided by the Marangoni effect. On the other hand, the change of viscosity with temperature showed no significant influence on the outcome of the droplet impact.

The axisymmetric impingement of solidifying molten solder droplets onto smooth metallic substrates in a microgravity environment is investigated numerically to provide basic information of the heat and fluid flow phenomena and to determine the governing parameters of the process. The numerical predictions are also tested against experimental data. Millimeter-sized droplet impact events in reduced gravity are employed for scale up modeling of the impingement of picoliter size droplets of molten eutectic 63%Sn-37%Pb solder used in electronic chip packaging. The present article reports on both numerical (the main focus of the paper) as well as experimental work. To this end, the employed numerical model considers the axisymmetric impact and subsequent solidification of an initially spherical, molten solder droplet on a flat, metallic substrate. The laminar Navier-Stokes equations, combined with the energy transport equations are solved simultaneously in the liquid region (melt) using a Lagrangian approach. In the solid (substrate and solidified droplet material) the heat conduction equation is solved. A time and space averaged model of the thermal contact resistance between the impacting droplet and the substrate, is also incorporated in the model. The numerical model is solved using a Galerkin finite element method, where a deforming, adaptive triangular-element mesh is employed to accurately simulate the large-domain deformations caused by the spreading and recoiling of the impinging droplet. The experimental work has been conducted in reduced gravity with technically relevant impact velocities of ∼0.2m/s, in order to provide validation of the numerical predictions. In reduced gravity of 2×10−4g to 5×10−4g, the impact conditions correspond to Re = O(100), We = O(1), and Fr = O(10000), Ca = O(0.001). Presentation of the numerical results in terms of the Froude and the Ohnesorge numbers aided their interpretation Among the results that stand out is the formation of a large number of frozen ripples on the droplet surface as a result of the simultaneous manifestation of rapid oscillations and solidification. Furthermore, a non-intuitive behavior of the solidification times is reported. To this end, the dependence of the final solidification time on the Froude number (representing the variation of the impact velocity) was not monotonic and featured a distinct minimum for all values of Ohnesorge numbers in this study. Despite the complexity of the phenomenon, the numerical model captures well the main features of the experimental results. In addition, it gives key insights on the influence of the Ohnesorge and Froude numbers on the solidification process.