The ability of metal alloys to rapidly oxidize in ambient condition presents both a challenge and an opportunity. Herein, we focus on opportunities buried in the passivating oxide of liquid metal particles. Recently described sub-surface complexity and order present an opportunity to frustrate homogeneous nucleation hence enhanced undercooling. Plasticity of the underlying liquid metal surface offers an autonomously repairing sub-surface hence the lowest E0 component dominates the surface unless stoichiometrically limited. This plasticity provides an opportunity to synthesize organometallic polymers that in situ self-assemble to high aspect ratio nanomaterials. An induced surface speciation implies that under the appropriate oxidant tension, the oxide thickness and composition can be tuned, leading to temperature-dependent composition inversion and so-called chameleon metals. The uniqueness of demonstrated capabilities points to the need for more exploration in this small but rather complex part of a metal alloy.

organic materials have diametrically opposite surface energies [1] hence, organics do not readily adhere to metals. [5] This paradoxical juxtaposition, however, can be overcome through scaling, [6] selfassembly, [7] and mechanical bonds. [8] We infer that non-Hertzian behavior of soft granular matters [9] combined with capillary self-assembly, [7] self-filtration, [7b,10] and jamming [10b,11] can lead to conformal mechanical bonds across dissimilar materials. Where the granular matters are undercooled liquid metal particles, sintering and solidification enable heatfree fabrication of conformal physisorbed (removable) conductive traces on virtually any textured or low modulus surface. [12] Deposition of solvent-suspended polydisperse metal particles onto textured surfaces forms a self-sorted tightly packed sediment through solvent evaporation-driven capillary self-assembly [7b] combined with selffiltration. [10b] Self-filtration is the process by which relatively large particles will clog or "jam" when passing through pores/fissures (Figure 1a). [10b] Solvent evaporation begins at the gas-liquid interface then penetrates through the self-filtered larger particles (Figure S1 and discussion in the Supporting Information). For soft deformable particles the drying process induces capillary-driven packing resulting in higher densification than in non-deformable analogues. [13] This process leads to reversible convergence of the particle ensemble stress field (Figure S1c, Supporting Information). Solvent evaporation also generates a capillary pressure gradient allowing smaller particles to pass through gaps formed by jammed larger ones (Figure S1a,b, Supporting Information). [10a] For the phenomena to occur, particle size polydispersity (largeto-small particle diameter) needs to be on the order of 1:3-7 (Figure S1, Supporting Information). [7b,10b,14] With decreasing dimensions, or over multi-scale pore dimensions, this process repeats ad infinitum leading to an autonomous size-differentiated packing of polydisperse slurries. Capillary self-assembly combined with self-filtration ensures that these particles are immobilized (jammed), creating a multi-layer self-locking particle bed (Figure 1a,b). [7,10b,14] The amount of self-filtration can be approximated through a pressure-dependent relationship (Equation (1), details in the Supporting Information) [10a]

Fabrication of bio‐templated metallic structures is limited by differences in properties, processing conditions, packing, and material state(s). Herein, by using undercooled metal particles, differences in modulus and processing temperatures can be overcome. Adoption of autonomous processes such as self‐filtration, capillary pressure, and evaporative concentration leads to enhanced packing, stabilization (jamming) and point sintering with phase change to create solid metal replicas of complex bio‐based features. Differentiation of subtle differences between cultivars of the rose flower with reproduction over large areas shows that this biomimetic metal patterning (BIOMAP) is a versatile method to replicate biological features either as positive or negative reliefs irrespective of the substrate. Using rose petal patterns, we illustrate the versatility of bio‐templated mapping with undercooled metal particles at ambient conditions, and with unprecedented efficiency for metal structures.

Undercooling metals relies on frustration of liquid–solid transition mainly by an increase in activation energy. Passivating oxide layers are a way to isolate the core from heterogenous nucleants (physical barrier) while also raising the activation energy (thermodynamic/kinetic barrier) needed for solidification. The latter is due to composition gradients (speciation) that establishes a sharp chemical potential gradient across the thin (0.7–5 nm) oxide shell, slowing homogeneous nucleation. When this speciation is properly tuned, the oxide layer presents a previously unaccounted for interfacial tension in the overall energy landscape of the relaxing material. We demonstrate that 1) the integrity of the passivation oxide is critical in stabilizing undercooled particle, a key tenet in developing heat‐free solders, 2) inductive effects play a critical role in undercooling, and 3) the magnitude of the influence of the passivating oxide can be larger than size effects in undercooling.