The central problem the team set out to solve is deceptively simple. Metallic lattice structures, in which thin struts form an open, interconnected framework, can be extraordinarily light. Some have densities less than one-tenth that of water, which in theory should make them ideal candidates for flotation. In practice, however, those open spaces are their undoing. Water pours freely through the interconnected pores of an open-cell lattice, filling every void until the structure loses whatever buoyancy its low density promised. According to lead researcher Dr Jordan Noronha of RMIT’s Centre for Additive Manufacturing, this fundamental flaw has made strong, lightweight metallic lattices unsuitable for marine applications, until now.

The RMIT team’s solution lies in a selective filling strategy. Rather than sealing the entire lattice or coating it in a waterproof shell, the engineers filled only the hollow titanium struts themselves with polyurethane foam, leaving the external openings of the structure unobstructed. The result is a hybrid material that allows seawater to flow through its open architecture while the sealed, foam-filled channels inside the struts retain their buoyancy. Crucially, the researchers validated this sustained flotation with samples that floated in freshwater for more than two months, demonstrating that the effect is not a fleeting laboratory curiosity but a durable property of the material itself.

Underpinning the achievement is a conceptual innovation in how engineers calculate whether an open structure will float. Conventional density measurements include all the open space within a lattice, even though water can occupy that space and it therefore contributes nothing to buoyancy. The RMIT team developed a new measure, which they call skeletal density, that considers only the parts of the structure that actually exclude water: the solid titanium walls and the sealed, foam-filled channels within the struts. This reframing yields an elegantly simple design rule. As Noronha explained, if the skeletal density of a structure is lower than that of the surrounding liquid, the structure will float, even when water flows freely through all of its external openings.

The mechanical performance of the new material is equally striking. When compared at the same overall density, the titanium hybrid lattice proved 70 percent stronger than the stainless steel or high-density polyethylene that is widely used in marine construction today. That comparison matters, because the plastics and steels currently deployed in jetties, buoys and floating sensor platforms represent the benchmark that any new material must beat, not merely match. A lattice that combines the corrosion resistance of titanium with strength well beyond these incumbent materials, while also floating, occupies a genuinely new position in the design space of marine engineering.

Corrosion, the perennial enemy of anything placed in the ocean, was addressed in short-term testing using natural seawater drawn from Melbourne’s Port Phillip Bay. After two weeks of immersion, the lattice lost only 0.15 percent of its mass, and its strength declined by less than 1 percent. While the researchers themselves describe this as short-term testing and point to long-term performance under realistic marine and deep-sea conditions as a next step, the early results suggest that the titanium framework can withstand the chemically aggressive environment that degrades so many conventional marine materials.

Perhaps the most remarkable property of the hybrid lattice is its resilience under damage. The structure remained buoyant even after significant cracking, failure at key connection points and the fracture of an entire lattice layer. It sank only after being severely crushed and compacted. The secret lies in the microstructure of the polyurethane foam: tiny, sealed cells within the foam trap gas and prevent water from flooding the hollow struts. In this way the foam acts as a distributed barrier against flooding, unlike conventional hollow marine structures, which can rapidly fill with water and lose flotation the moment their walls are breached. For safety-critical applications such as floating platforms and navigation buoys, this damage tolerance could prove decisive.

The team moved beyond laboratory coupons to demonstrate the technology at a functional scale with a 3D-printed marine buoy. Placed in a turbulent seawater tank that was rotated up to 45 degrees, the buoy remained stable without needing a sealed casing, a protective coating or any additional flotation aid. That the device survived these conditions with no supplementary buoyancy measures underscores the practicality of the skeletal-density approach: the flotation is intrinsic to the material’s architecture rather than bolted on as an afterthought.

Project leader Distinguished Professor Ma Qian said the next steps include scaling up the demonstration parts and testing long-term performance under realistic marine and deep-sea conditions. He also emphasised that the structure is highly tailorable, and the group is open to exploring applications well beyond marine infrastructure. By changing the material inside the titanium framework, he noted, a similar structure could be tailored for energy absorption, thermal management, vibration control and other uses. This versatility reflects a broader trend in metamaterials research, where the geometry of a structure, rather than its composition alone, dictates properties that no conventional solid material can offer.

The research was led by RMIT’s Centre for Additive Manufacturing in collaboration with the Conservatoire National des Arts et Métiers in France, with support from the Australian Research Council and RMIT’s School of Engineering. The study, titled Breaking the surface: buoyant metal–polymer open–cell hybrid lattice metamaterials, was published in Advanced Materials on 28 August 2026. For a field in which the ocean has always demanded a trade-off between strength and weight, the arrival of a titanium structure that floats, survives damage and shrugs off seawater marks a genuinely new chapter, one that engineers of jetties, buoys and deep-sea systems will be watching closely.

Subject of Research: Buoyant metal–polymer hybrid lattice metamaterials for marine infrastructure

Article Title: Engineers create world-first floating titanium

Article References: Engineers create world-first floating titanium. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: titanium lattice, metamaterial, 3D printing, buoyancy, marine infrastructure, polyurethane foam, skeletal density, corrosion resistance, RMIT University, Advanced Materials, additive manufacturing, floating structures

The terahertz regime, which sits between microwaves and infrared light on the electromagnetic spectrum, has long promised transformative applications because it is non-ionizing, interacts distinctively with molecular and biological systems, and offers enormous bandwidth. Yet building compact, efficient, and broadband functional components in this range remains a formidable engineering challenge. Metamaterial absorbers, which use carefully engineered resonant structures to capture electromagnetic energy at subwavelength scales, have emerged as leading candidates. Conventional metallic versions, however, tend to suffer from limited bandwidth, weak tunability, and high sensitivity to geometric variations, particularly when multiple resonant modes are needed to cover a wide spectrum.

Graphene offers a way around these constraints. The atomically thin carbon sheet supports strongly confined surface plasmon polaritons, and its optical response can be actively controlled by adjusting its Fermi level through electrostatic gating or chemical doping. This means the resonance frequency and absorption characteristics of a graphene device can be modulated reversibly without changing its physical geometry. Earlier graphene-based terahertz absorbers relied on multilayer stacks, arrays of ribbons of different widths, or hybrid integration with phase-change materials such as vanadium dioxide. While effective, these approaches often demand complex fabrication, precise alignment, or dual tuning mechanisms, and thermally activated phase-change designs introduce challenges related to switching speed and power consumption.

The new design, developed by Hamzeh Sam Daliri, Hamed Saghaei, Hadi Dehbovid, and Reza Yousefi, consists of a three-layer metal-dielectric-graphene architecture. At the top sits a patterned graphene metasurface: a central multi-tip element inspired by the maple leaf, flanked by two rectangular graphene strips, together evoking the Canadian flag. The central pattern spans approximately 2.4 by 1.0 micrometers, with a patterned graphene area of about 0.84 square micrometers, while the lateral strips measure 1.25 by 2.75 micrometers. Beneath the graphene lies a dielectric spacer 6 micrometers thick with a refractive index of 1.4, and at the bottom a continuous gold film 1 micrometer thick acts as a back-reflector that suppresses transmission entirely.

The novelty lies not in the motif itself but in its functional plasmonic topology. The multi-tip central element and the lateral strips introduce multiple characteristic current-path lengths, sharp edges, and high-curvature regions within a single patterned plane. These features excite several interacting plasmonic modes whose controlled spectral overlap produces broadband absorption without vertically stacked resonators or hybrid material systems. The researchers traced the design’s evolution step by step: a single central resonator produced a narrow response, adding the two lateral strips broadened it by introducing additional current paths, and replacing the simple resonator with the multi-tip maple-leaf pattern further enhanced bandwidth through localized charge accumulation at its edges and tips.

The team simulated the structure using a three-dimensional finite-difference time-domain method, launching a broadband plane wave from 0.5 to 10 terahertz and applying periodic boundary conditions to model an infinite metasurface. Because the gold backplane eliminates transmission, absorptance reduces simply to one minus reflectance. Graphene was modeled as an atomically thin conductive sheet using the Kubo formalism, with an effective conductivity scaling factor representing multiple electronically decoupled sheets. An equivalent-circuit interpretation, in which each current path behaves as a lossy series RLC branch coupled to a grounded dielectric transmission-line section, explains why near-unity absorption occurs when the input impedance of the structure approaches that of free space, allowing incident energy to be fully dissipated in the graphene’s finite resistance.

A central finding of the parametric study is that absorption depends non-monotonically on graphene conductivity. For a single graphene layer, raising the chemical potential from 0.2 to 0.6 electronvolts boosted peak absorption from about 62 percent to 98 percent at 5.2 terahertz, close to a critical-coupling condition where impedance is optimally matched to free space. Pushing further to 0.8 electronvolts caused overcoupling, impedance mismatch, and a drop to roughly 85 percent. Similarly, increasing the effective number of graphene layers helped at low Fermi energies but degraded performance at higher ones: at 0.1 electronvolts, three- and four-layer structures achieved broadband absorption near 97 percent, whereas at 0.4 electronvolts and above, one- or two-layer configurations were clearly superior, with thicker stacks suffering detuning and destructive interference.

The dielectric spacer and lateral geometry proved equally consequential. Spacer thicknesses between 6 and 10 micrometers generally yielded absorption above 90 percent, while thinner cavities could not support well-developed resonant modes and thicker ones entered a regime of mode saturation and phase accumulation. For one optimized configuration with four effective graphene layers and a chemical potential of 0.6 electronvolts, an 8-micrometer spacer produced absorption approaching 100 percent across 4.8 to 8.2 terahertz. Low-index dielectrics such as polymethylpentene, PTFE, TOPAS, and PDMS emerged as suitable spacer candidates, and increasing the refractive index generally redshifted resonances while reducing peak absorption. Strip width sweeps showed that widths near 1.25 to 1.5 micrometers maximize plasmonic confinement at low-to-moderate chemical potentials, while narrower strips fail to sustain strong resonances.

Electric-field maps confirmed the physical picture: the strongest field confinement and energy concentration occurred in the 4 to 6 terahertz range, with pronounced hotspots along the edges, tips, and high-curvature regions of the maple-leaf pattern, precisely where charge accumulates and plasmonic modes localize. The authors emphasize that the high absorption should not be confused with the roughly 2.3 percent single-pass absorption of isolated monolayer graphene; here, plasmonic patterning, cavity enhancement, a reflective ground plane, and impedance matching combine to multiply light-matter interaction dramatically. Compared with prior broadband absorbers, the design meets a stricter 95 percent absorption criterion over a 4-terahertz interval within a single patterned plane, avoiding both stacked resonator architectures and thermally activated phase-change materials.

The researchers position the device as a performance-complexity compromise rather than an absolute bandwidth record, and its practical significance lies in the transferable design guidelines it establishes: electronic conductivity, resonator geometry, and cavity parameters must be jointly optimized, since excessive values of any single parameter transform the absorber into a reflector. With electrical tunability achieved through gate voltages that set the graphene chemical potential, and with compatibility with planar fabrication processes, the flag-inspired metasurface points toward reconfigurable terahertz systems for sensing, imaging, filtering, electromagnetic shielding, and future wireless communication networks.

Subject of Research: Electrically tunable broadband terahertz absorption using a Canada-flag-inspired patterned graphene metasurface

Article Title: Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface

Article References: Sam Daliri, H., Saghaei, H., Dehbovid, H., & Yousefi, R. (2026). Tunable broadband terahertz absorber based on a Canada-flag-inspired graphene metasurface. Results in Optics, 25, Article 101157. https://doi.org/10.1016/j.rio.2026.101157

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101157

Keywords: terahertz absorber, graphene metasurface, surface plasmon polaritons, broadband absorption, electrical tunability, impedance matching, FDTD simulation, metamaterial, plasmonic resonance, terahertz sensing, dielectric spacer, Kubo formalism