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The synthesis of PVA hydrogels (PVA-Hy) requires a highly basic environment ( e.g. , an aqueous solution of sodium hydroxide, NaOH, 14% w/w, 4.2 M), but the rapid crosslinking of PVA due to high pH makes it challenging to perform layer-by-layer three-dimensional (3D) printing of PVA-Hy. This work demonstrated 3D printing of PVA-Hy in moderate alkaline conditions ( e.g. , NaOH, 1% w/w, 0.3 M) assisted by aqueous two-phase system (ATPS). Salting out of PVA to form ATPS allowed temporal shape retention of a 3D-printed PVA structure while it was physically crosslinked in moderate alkaline conditions. Crucially, the layer-to-layer adhesion of PVA was facilitated by delayed crosslinking of PVA that required additional reaction time and overlapping between the layers. To verify this principle, we studied the feasibility of direct ink write (DIW) 3D printing of PVA inks (5-25% w/w, μ = 0.1-20 Pa s, and MW = 22 000 and 74 800) in aqueous embedding media offering three distinct chemical environments: (1) salts for salting out ( e.g. , Na 2 SO 4 ), (2) alkali hydroxides for physical crosslinking ( e.g. , NaOH), and (3) a mixture of salt and alkali hydroxide. Our study suggested the feasibility of 3D-printed PVA-Hy using the mixture of salt and alkali hydroxide, demonstrating a unique concept of embedded 3D printing enabled by ATPS for temporary stabilization of the printed structures to facilitate 3D fabrication.

Droplets adhere to surfaces due to their surface tension γ and understanding the vertical force F d required to detach the droplet is key to many technologies ( e.g. , inkjet printing, optimal paint formulations). Here, we predicted F d on different surfaces by numerically solving the Young-Laplace equation. Our numerical results are consistent with previously reported results for a wide range of experimental conditions: droplets subjected to surface vs. body forces with | F d | ranging from nano- to milli-newtons, droplet radii R ranging from tens of microns to several millimetres, and for various surfaces (micro-/nano-structured superhydrophobic vs. lubricated surfaces). Finally, we derive an analytic solution for F d on highly hydrophobic surfaces and further show that for receding contact angle θ r > 140°, the normalized F d /π R is equivalent to the Young-Dupre work of adhesion γ (1 + cos  θ r ).