Evaporative cooling operates across scales. Mammals use it for thermoregulation through sweating. Steven Chu and colleagues exploited it to cool trapped atoms to near absolute zero, earning them a Nobel Prize. In our setup, evaporative cooling drives the freezing of water in a micrometer-scale jet. We were spraying water in vacuum to get rid of air drag, and noticed that the spray droplets froze. Putting a 3D printer in the vacuum chamber with the jet nozzle as the printing head allowed us to print an ice christmas tree.
The mechanism is straightforward. At very low pressure, water molecules at the liquid surface escape continuously as vapor. Each departing molecule carries the latent heat of vaporization, thus cooling the water jet. The very fine jet we use for printing has a very high surface-to-volume ratio, making heat extraction very efficient: the bulk liquid cools rapidly, dropping tens of Kelvin over a fraction of a second. When the jet reaches the substrate or a previously deposited ice layer, it freezes just after its impact.
This principle is not new, but its application to 3D printing is. Previous ice-printing methods relied on cooled substrates or cryogenic infrastructure (liquid nitrogen, helium). Our approach integrates the jet into a commercial 3D printer housed inside a transparent vacuum chamber. The printer's motion control guides the water jet layer-by-layer, building geometry on demand. Operating costs are minimal compared to cryogenic systems. The printed ice is pure—no dopants, no supporting material, no foreign particles. For biomedical and microfluidic applications, this purity directly affects cell biocompatibility and fluid transport properties
Inside our vacuum chamber, a 16-micrometer water nozzle (fed by an HPLC pump) is mounted on a 3D printer. When activated, it ejects a water jet at a few meters per second. The printer's XY stage moves at 20 mm/s, depositing the jet droplets in its path. Each droplet lands on the growing ice structure below. This is where the freezing delay becomes critical: the deposited water remains liquid for approximately 0.5 seconds before fully solidifying. During this half-second window, multiple droplets that have formed from the jet converge into a coherent line. Surface tension holds them together. Then, suddenly, crystallization begins and propagates through the entire layer. The result is a solid ice line with consistent dimensions. Layer by layer, the 3D printer builds the complete structure.
This freezing delay is key. It enables overhanging features that conventional 3D printing cannot achieve without support material. By reducing the printer speed, we printed tilted struts at 48° from horizontal—free-standing cantilevers that remain stable. We also printed vertical pillars and zigzag formations. When the vacuum chamber was shaken, these slender ice structures oscillated elastically without breaking, demonstrating mechanical strength.
The pure ice structures can serve as sacrificial templates. A branching ice form printed by the 3D printer can be cast in resin or polymer, then melted away to reveal hollow channels. For tissue engineering, this produces scaffolds with designed porosity. For microfluidics, it enables custom fluid networks. Once the print is complete and the vacuum is released, the ice melts cleanly to water—no residue, no post-processing waste.
The vacuum requirement opens possibilities beyond Earth. Mars has a surface pressure of 6 mbar, within the operating range. A similar 3D printer on Mars could print structures from local water ice using the same evaporative cooling principle, without importing cryogenic infrastructure.
The full analysis appears in our arXiv preprint: "An Ice Christmas Tree: Fast Three-Dimensional Ice Printing via Evaporative Cooling in Vacuum". We show the printing of other structures, high-speed imaging of the deposition process, quantification of the ice formation zone, and dimensional measurements demonstrating reproducibility across multiple prints.
REFERENCE: Demmenie, M., Kooij, S., & Bonn, D. An Ice Christmas Tree: Fast Three-Dimensional Ice Printing via Evaporative Cooling in Vacuum. arXiv:2512.14580 (2025).