(Nanowerk Spotlight) The ability to mold and weld materials into desired shapes and structures is a fundamental manufacturing capability that enables the creation of countless products we rely on every day. For centuries, this process has been limited to solid materials like metals, plastics, and ceramics. Liquids, due to their tendency to flow and adopt a spherical shape to minimize surface energy, have been far more challenging to structure and manipulate.
However, recent advances in nanotechnology and interfacial science are starting to change that. By assembling and jamming nanoparticles at liquid-liquid interfaces, pioneering researchers have shown it is possible to “armor” liquid surfaces and mold them into stable non-spherical shapes. This has opened up a new field of structured liquids with potential applications ranging from chemical reactors to soft robotics.
Still, significant limitations have persisted. The liquid structures created have typically been irreversible and their geometries constrained by the original molds used to shape them. The inability to reconfigure shapes or weld pre-formed components together has restricted the complexity and functionality that could be achieved.
Now, in a significant step forward, a research team led by Shaowei Shi at Beijing University of Chemical Technology has developed a new approach leveraging temperature-responsive nanoparticle surfactants that allows reversible structuring and, for the first time, thermal welding of liquids. The work, published in the journal Advanced Materials (“Thermal Welding of Liquids”), opens up new possibilities for the modular construction of all-liquid fluidic devices.
Schematic representation showing a) the interfacial-energy-reduction induced dynamic exchange and temperature-stimulated reversible assembly of nanoparticle surfactants at the oil–water interface, and b) the mechanical reconfiguration and thermal welding of liquids with jammed nanoparticle surfactants at the oil–water interface. (Image: Reprinted with permission by Wiley-VCH Verlag)
The key innovation is the use of nanoparticle surfactants (NPSs) that dynamically attach to liquid-liquid interfaces via temperature-sensitive interactions. The researchers synthesized three types of NPSs using star polymers with cyclodextrin cores that can host-guest complex with benzyl-terminated poly(L-lactide) ligands in the oil phase. By tuning the type of cyclodextrin used (α, β, or γ), the binding strength and interfacial activity of the NPSs could be precisely controlled.
At room temperature, the NPSs rapidly adsorb to the interface and jam together to form a stiff film that can lock liquids into non-equilibrium shapes imposed by an external mold or mechanical force. Remarkably though, when NPSs with different binding strengths are mixed, a dynamic exchange occurs as the system minimizes interfacial energy, displacing weaker NPSs with stronger variants.
Raising the temperature above a critical point weakens the NPS host-guest complexes, causing them to disassemble and un-jam in a process reminiscent of solid glass transitioning to a viscous liquid state. This allows the structured liquids to relax back to a spherical shape. Importantly though, the kinetics of this jamming-to-unjamming transition are slow, taking minutes at elevated temperature. This sluggish response provides a practical time window to manually manipulate and reconfigure the softened liquid structures before they irreversibly lose their jammed state upon cooling. The researchers leveraged this key property to achieve a fundamentally new capability – welding of structured liquids.
Real-time demonstration of reshaping a macroscopic spherical water droplet into complex structures using external forces. This video showcases the application of needles and tweezers to mold the water droplet into the letter W, highlighting the robust interfacial jamming of nanoparticle surfactants that maintain the shapes against surface tension.
The researchers leveraged this dynamic yet sluggish thermal response to achieve a fundamentally new capability – welding of structured liquids. By raising the temperature of two liquids with NPS-jammed interfaces pressed into contact, they were able to join them together into a single merged object that retained its welded shape upon cooling. The size of the junction could be tuned by controlling the heating time.
Using this thermal welding technique, the team demonstrated the ability to modularly construct complex liquid structures like branched tubules and even Mickey Mouse-inspired shapes with internally connected compartments. Mixing of dyes injected into the structures confirmed the formation of continuous fluid channels.
To showcase the ultimate potential of the NPS system, Shi and colleagues fabricated reconfigurable all-liquid fluidic devices. Through a cut-and-weld process, they were able to link together and rearrange liquid tube segments into arbitrary flow circuits, directing injected dye solutions through different channel paths.
The ability to thermally weld structured liquids and reconfigure them on-demand in a modular fashion overcomes key limitations of previous liquid structuring approaches. It opens up new possibilities for the rational design and construction of all-liquid devices with complex geometries and functions.
Dynamic assembly of modular all-liquid fluidics through thermal welding. This video illustrates the process of selectively cutting and welding liquid tubule segments to redirect fluid flow within a branched channel structure. Witness how tailored thermal manipulation facilitates the precise control of liquid paths, embodying the potential for reconfigurable liquid circuit systems.
Potential applications span diverse fields. All-liquid chemical reactors with optimized microfluidic flow could enhance the efficiency and selectivity of multiple-step syntheses. Dynamically reconfigurable liquid electronic components like capacitors and resistors could enable tunable circuits. Modular assembly of liquid-based soft robots could allow them to repair damage or adapt their bodies situationally. Biomedical devices like dialysis cartridges and organ-on-a-chip platforms could potentially benefit from all-liquid designs with biomimetic vascularized architectures.
Major challenges still remain in translating this initial proof-of-concept into real-world devices. Issues of liquid miscibility, mechanical stability, NPS biocompatibility, and scalable large-volume manufacturing will need to be addressed. Still, the unprecedented capabilities demonstrated in this work represent a major stride towards a new paradigm of structured liquid engineering.
With further advancements in the chemistry of dynamic interfacial assemblies, structured liquids may one day rival the versatility of their solid counterparts. Elevating the humble liquid to a true state of matter for manufacturing stands to reshape the material landscape and usher in a new age of all-liquid machines. The emerging future Shi and team have brought into focus is one where the dividing line between liquid and solid disappears entirely – a world of shapeshifting, reconfigurable fluids with the power to flow, form and function on demand.
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