Physics-based nozzle design rules for high-frequency liquid metal jetting

Metal additive manufacturing (AM) is emerging as a viable alternative to traditional methods such as casting to make supply chains more resilient and cost-effective for small-batch, multi-variety, and spare parts. Drop-on-demand (DoD) liquid metal jetting stands out due to its high deposition throughput, low porosity, use of off-the-shelf materials (aluminum wire feed), predictable material properties, and operational safety. The process is characterized by ejecting a sequence of droplets from a microfluidic nozzle attached to the end of a pump where the metal is molten and pushed down at frequencies of a few hundred Hz using mechanisms ranging from magnetohydrodynamics to pneumatics. As every droplet is released, the meniscus (liquid-gas interface) at the tip of the nozzle oscillates due to the dynamic interplay between surface tension, the inertia of the fluid, and imposed pressure gradient. Due to viscous dissipation in the fluid, the oscillations are damped.

To enable fast and reliable/repeatable builds, it is important to produce droplets with consistent volume, shape, and velocity distributions, which, in turn, strongly depend on the rate at which the post-ejection energy in the liquid dissipates. One way to measure this rate is by observing the decay of the oscillations of the meniscus membrane at the tip of the nozzle. Ideally, the meniscus should settle before the next droplet starts forming.

The most common nozzle concepts are axisymmetric due to their ease of manufacturing (e.g., via micro-drilling). The resulting circular cross-sections lead to an inevitable tradeoff between droplet parameters, target throughput, and relaxation time. Nozzle with a smaller diameter has a shorter relaxation time, but it is hard to push the liquid through such nozzle due to increased resistance. As a result, a higher pressure gradient is required to meet the throughput target. An increased flow velocity may eject an elongated droplet that breaks apart after the ejection. The critical question is to design a nozzle that ejects a spherical droplet with target velocity and mass while minimizing the relaxation time of post-ejection oscillations. 

Collaborators: Morad Behandish (PARC), Svyatoslav Korneev (PARC), Christoper Somarakis (PARC), Adrian Lew (PARC/ Stanford)