Omnidirectional 3D printing of metal micro-architectures via droplet sliding over curved surfaces

As a simple, fast, and green 3D printing method, droplet-based manufacturing is suitable to print various materials to manufacture structures. However, up to now, it has hardly been capable of manufacturing omnidirectional designs with slender horizontal or upside-down sections, since the required rotation of the nozzle or workpiece negatively affects printing performance. Here, we employed a novel slide-guided droplet deposition method for printing parts with arbitrary-angle overhangs up to 180o. Arc-shaped slides with metallophobic surfaces are used to smoothly deflect droplets and provide a full control of droplets’ impact angle onto the workpiece. We show that gently curved impact surfaces prevent droplet bouncing, and quantitatively describe the transition from sliding to bouncing by measurements, simulations, and theory that predict our process window. Furthermore, the temperature of the slides is controlled to modify the temperature of droplets after they leave the generator, enabling fabrication of horizontal pillars with locally tunable morphology and optimization of drop-to-drop adhesion. The versatility of the slide-guided deposition is highlighted by fabricating structures with high aspect ratios and free-standing branches in arbitrary angles including hung sections where droplets are deposited from a fully reversed direction. Since the proposed slide-guided deposition uniquely facilitates fabrication of slender metal structures from arbitrary angles without rotating the workpiece or nozzle, it helps advance the development of many other research fields, such as antennas, branch-like heat sinks, and metal micro-lattices, where complex structures are needed.

Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces

Evaluation of superhydrophobic (SH) surfaces based on contact angle measurements is challenging due to the high mobility of drops and the resolution limits of optical goniometry. For this reason, some alternatives to drop-shape methods have been proposed such as the damped-oscillatory motion of ferrofluid sessile drops produced by an external magnetic field. This approach provides information on surface friction (lateral/shear adhesion) from the kinetic energy dissipation of the drop. In this work, we used this method to compare the low adhesion of four commercial SH coatings (Neverwet, WX2100, Ultraever dry, Hydrobead) formed on glass substrates. As ferrofluid, we used a maghemite aqueous suspension (2% v/v) synthesized ad hoc. The rolling magnetic drop is used as a probe to explore shear solid–liquid adhesion. Additionally, drop energy dissipates due to velocity-dependent viscous stresses developed close to the solid–liquid interface. By fitting the damped harmonic oscillations, we estimated the decay time on each coating. The SH coatings were statistically different by using the mean damping time. The differences found between SH coatings could be ascribed to surface–drop adhesion (contact angle hysteresis and apparent contact area). By using this methodology, we were able to grade meaningfully the liquid-repelling properties of superhydrophobic surfaces.