Absorption of picoliter droplets by thin porous substrates

Abstract

Absorption of picoliter (pL) droplets into porous substrates is studied experimentally and numerically. In the case of pL droplets, major phenomena involved in the interaction between droplet and porous media develop at different time scales: spreading and wetting at microseconds, absorption and wicking at milliseconds, and evaporation at seconds. Therefore, one can decouple these processes to minimize the complexity of the study. A high-speed imaging system capable of 1 million frames per second is used to visualize individual droplets impacting, spreading, and imbibing on substrates. To simulate droplet dynamics, the governing equations for flow outside and inside porous media are proposed and solved using an in-house developed computational fluid dynamics solver. The simulation results are in good agreement with the experimental data. The effect of drop impact velocity and fluid properties on final dot shape in the porous substrates is investigated through a series of parametric numerical studies.

Binder jetting additive manufacturing process fundamentals and the resultant influences on part quality.

“…6.7 a) would result in larger spreading diameter at the end of impact-driven spreading phase as previously explained. After the impact-driven spreading phase of the droplet ends, the capillary-driven spreading phase becomes the dominant mechanism [91,113,158]. In this stage, the powder beds with larger pore sizes facilitate the binder permeation into the porous structure due the improved permeability [95], which can be quantified by the Kozeny equation that correlates the permeability with the mean diameter of particles [177,178]:…”

“…Considering the common droplet jetting frequency (4 kHz), the droplet velocity and the prinrhead distance from the powder bed surface (approximately 500 µm), the time interval between the successive droplets impacting the powder bed is on the order of a few hundred micro-seconds. As the time for binder droplet depletion from the powder bed surface is on the order of milliseconds [91,95], on can expect that the droplets could coalesce and form continuous lines before the binder permeation into the powder bed is complete. Therefore, the generation of each cross section of an object in the BJ-AM process can be effectively treated as the formation of straight lines with various lengths as shown schematically in Fig.…”

“…After the impact, the droplets might undergo different phases driven by inertial forces, capillary forces, and gravitational forces [86,158]. Due to the droplet volume range and binder physical properties typically used in the binder jetting process, which result in the small Bond number (BO << 1), the effects of gravitational forces on the droplet dynamics can be ignored [86,91,95,158]. The dimensionless Bond number can be determined from the below equation:…”

“…For the BJ-AM system examined in the present investigation, the resulting Bond number is on the order of 10 -4 , which indicates that gravity influence on the droplet interaction with powder bed is negligible. Therefore, inertial and capillary forces can be considered as the dominant driving forces for spreading and penetration of liquid binder droplets into the powder bed [91,158]. From a physiochemical perspective, the interaction between a liquid droplet and a porous material is a very complex process [188].…”

One Dimensional Model for Droplet Ejection Process in Inkjet Devices

“…The fourth example is a DOD inkjet under a step driving pressure to check the overall performance of the 1D simulation. The numerical results are compared with established simulation data available from the literature and the high fidelity 2D axisymmetric simulations using our in-house developed code which has been used in modeling of various surface-tension driven flows in inkjet technologies [27,[50][51][52][53].…”