A microrobotic tool uses magnetic control and hydrophobic particles to cut water surfaces and create reconfigurable platforms for fluidic applications.
(Nanowerk Spotlight) Many of the most advanced tools in chemical analysis, biomedical engineering, and materials research rely on the ability to manipulate small amounts of liquid with speed and precision. Whether isolating cells for analysis, initiating chemical reactions on demand, or generating controlled gradients of nutrients or drugs, scientists depend on liquid systems that can be shaped, segmented, and directed with minimal waste and maximal flexibility.
This demand is especially pronounced in open systems, where rigid containers and fixed microchannel geometries are either impractical or limiting. Yet water—the most commonly used liquid—resists free-form structuring in these settings. Its strong surface tension and cohesive flow properties cause it to quickly heal any incisions or deformations. Conventional tools, even those designed with hydrophobic coatings or bioinspired geometries, cannot draw stable lines, compartments, or shapes across an unconfined water surface.
This limitation has hindered efforts to create reconfigurable fluidic platforms for diagnostics, soft robotics, and high-throughput assays. Existing systems either require enclosed channels that are prone to clogging and inflexible by design, or rely on complex external fields and chemical patterning to exert control. A robust, mechanically controlled way to shape water in open space could enable an entirely new class of fluidic devices.
In a study published in Advanced Materials (“Magnetically Controlled Mechanical Cutting of Water”), researchers at Xi’an Jiaotong University report a strategy that allows water to be mechanically cut and stably patterned without the use of rigid containers.
Their approach relies on coating the surface of shallow water layers with hydrophobic particles, forming a thin encapsulating barrier that suppresses water’s self-healing behavior. Using a magnetically controlled steel sphere coated in a hydrophobic material, they create stable grooves and compartments in the coated water, effectively turning the liquid into a reconfigurable medium for fluidic operations.
At the core of the method is the transformation of water into a stabilized interface known as hydrophobic particle-encapsulated water (HPEW). The researchers use silica nanoparticles, paraffin particles, or PTFE particles to form this layer. When a hydrophobic sphere presses into the surface of HPEW, the hydrophobic particles redistribute, creating a physical depression that resists closure. These particles are carried by the flow induced during the cutting process, which helps maintain the incision.
The study shows that water thickness is a critical parameter. When the water layer exceeds one millimeter, internal hydrostatic pressure increases, pushing against the formation of stable cuts. The authors used a combination of mechanical modeling and finite element analysis to explain this effect. The deformation of the water surface and the redistribution of hydrophobic particles are both governed by fluid mechanical forces and surface energy considerations. If the stored strain energy in the particle layer exceeds the energy required to create a new water interface, a stable cut forms. Otherwise, the interface quickly returns to its original state.
To achieve programmable and precise movement, the team developed a magnetic microrobot system. A magnet moves along a rail beneath a transparent PMMA platform, guiding the hydrophobic sphere along defined trajectories on the water surface. The magnet exerts both downward and forward forces, ensuring that the sphere maintains contact with the water and follows the intended cutting path. Using this setup, the researchers demonstrated the ability to trace arbitrary shapes and letters into the water surface, creating temporary but stable millifluidic chips.
These open millifluidic chips (OMCs) differ from conventional microfluidic devices by eliminating the need for enclosed channels. Liquids can be injected, mixed, aspirated, or separated simply by applying or removing pressure at specific regions of the cut pattern. The open nature of the system also avoids common issues such as channel clogging or bubble formation. By cutting specific patterns into HPEW, the researchers created interconnected compartments for solution handling.
For example, they demonstrated passive mixing by injecting a dye into one reservoir and observing its movement into adjacent channels. They also achieved active mixing using suction to draw liquids from multiple compartments into a central region.
This video shows a surprising way to shape and control water—without using any walls, containers, or printed channels. What you’re seeing are three compartments and connecting paths formed directly in a thin layer of water coated with hydrophobic particles. These features are not printed, drawn, or enclosed by physical barriers. Instead, they are created by mechanically dragging a hydrophobic sphere across the surface, displacing the particle layer and forming stable, recessed grooves. When blue dye is added to the center compartment, it flows on its own into the two outer compartments through the pre-cut channels. No pumps, valves, or physical structures are involved. The liquid stays sharply confined to the shapes cut into the water itself, held in place by surface tension and particle displacement.
To further showcase the flexibility of the platform, the researchers implemented electrophoresis in the OMCs. Using electrical fields, they were able to separate proteins and dyes in open water channels. They also performed colorimetric assays to detect pollutants such as chromate ions in river water, achieving results comparable to standard spectrophotometry. The OMCs proved compatible with other detection methods as well, including test strips and fluorescence imaging.
The same cutting method was used for chemical synthesis. The team synthesized gold nanoparticles within an OMC by mixing chloroauric acid with reducing agents. They also performed a silver mirror reaction directly on the hydrophobic particle layer, using it as a substrate for material deposition. Additionally, they shaped conductive hydrogels containing graphene into specific geometries, then confirmed their functionality by incorporating them into an electrical circuit. These examples illustrate that OMCs can function not just as fluidic processors but also as miniaturized chemical reactors.
The platform supports 3D cell culture as well. In one experiment, the team patterned chambers containing different concentrations of vascular endothelial growth factor (VEGF) to study how human endothelial cells respond to biochemical gradients. The results showed clear variation in vessel-like structure formation depending on VEGF concentration. In another test, they created a ring-shaped OMC to pattern a network of cells, demonstrating potential applications in tissue engineering and organ-on-chip systems.
By making water itself the substrate for fluidic control, this work introduces a new class of liquid-phase microengineering. The use of hydrophobic particles to stabilize thin water layers, combined with magnetically guided mechanical tools, enables free-form structuring and reconfiguration. The OMCs offer a versatile, open alternative to rigid microfluidic chips and can be adapted for sensing, synthesis, and biological studies. As the authors suggest, further automation and extension to other liquids could expand the platform’s utility in industrial and research settings.
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