Researchers have developed a portable technique that assembles and concentrates micro- and nanoscale dispersoids—including bacteria—in liquid using gold-coated optical fibre tips. When you shine a near-infrared laser on these tips, they generate photothermal bubbles and set up Marangoni convection, pulling particles toward the space between the bubble and the tip.
This process creates three-dimensional optical condensation away from a substrate. It significantly boosts assembly efficiency compared to conventional flat substrates and works across a wide range of particle concentrations.
That opens up new possibilities for rapid bioanalysis, diagnostics, and targeted delivery. Right now, the method needs pretty high laser power at a non-resonant wavelength, but researchers are working on plasmonic optimization to cut down on power and thermal load.
Device design and operating principle
The core of the system is a commercial optical fibre with a thin 10 nm gold film coating the tip. When you hit it with a 976 nm CW laser at about 390 mW, the tip acts as a localized light and heat source.
This creates steam-like bubbles at the tip, which then drive strong Marangoni convection in the surrounding liquid. These flows pull dispersoids from both horizontal and vertical directions toward the narrow region between the bubble and the fibre tip.
That enables three-dimensional optical condensation well above the substrate. In contrast, a flat gold-coated substrate just doesn’t have the same vertical transport or viscous-damping effects, so its assembly efficiency is much lower.
Bubble-driven flow and three-dimensional condensation
Experiments show that particles—including 1 µm polystyrene beads—stick to bubble surfaces and sometimes get carried away if the bubbles detach. Surfactant dynamics play a role here, affecting bubble stability after you switch off the laser.
The flows converge on the fibre tip, creating a compact assembly site in mid-air. This lets you concentrate particles with high contrast, without needing a substrate to trap them.
Key findings and computational support
Compared with flat-substrate coatings, fibre-based modules achieved much higher assembly efficiency. They reached up to about 11.6% at low particle concentrations (4.55 × 10^6 to 4.55 × 10^8 particles/mL), while the flat substrate managed only about 0.9%.
This boost in efficiency comes from vertical transport and reduced viscous damping, which speed up particle movement and expand the collection volume. The method concentrated not just polystyrene particles but also nanoparticles and bacteria, showing it works for many types of micro- and nanoscale dispersoids.
Finite-element simulations of temperature and flow fields matched the observed convective patterns. These results reinforce the idea that strong, asymmetric Marangoni flows converge on the fibre tip to form a low-velocity assembly site between the bubble and tip.
Role of simulations and interface dynamics
The simulations back up a mechanism where localized heating drives surface-tension–driven currents that funnel particles to the bubble–tip region. When the fibre touches the substrate, you see extra lateral migration along the fibre–substrate interface, probably due to a mix of convection, thermophoresis, and capillary forces.
Flat-substrate setups don’t show this effect. This mix of forces helps concentrate particles at a defined, substrate-adjacent–free zone, which could reduce fouling and make downstream analysis cleaner.
Practical considerations, challenges, and future directions
The current setup uses fairly high laser power at a non-resonant wavelength, but the authors think plasmonic optimization of the nanostructure could lower power needs and cut down on thermal damage. The technique’s portability and improved transport efficiency make it promising for rapid bioanalysis, diagnostics, and drug delivery—especially when you need to concentrate micro- and nanoscale dispersoids gently.
Ongoing work aims to reduce power, integrate the device with microfluidic platforms, and expand it to more types of particles and biological species. It’s a work in progress, but the potential here is pretty exciting.
- Applications—rapid concentration for sensing, diagnostics, and point-of-care testing
- Mechanistic insights—finite-element–backed understanding of bubble-driven convection and Marangoni flows
- Optimization goals—lower laser power, reduced thermal load, and plasmonic design improvements
Conclusion: toward faster, more sensitive micro- and nanoscale analytics
Researchers paired a gold-coated optical fibre tip with a near-infrared laser. This setup generates photothermal bubbles and creates Marangoni convection.
With this method, they managed to assemble and concentrate micro- and nanoscale dispersoids in three dimensions. It’s a clever approach that could make portable bioanalysis and diagnostic workflows more practical.
There’s hope that upcoming tweaks will cut down on power requirements and let the technique work with more types of fluids, particles, and biological samples. If that happens, the possibilities might get even more interesting.
Here is the source article for this story: Highly efficient three-dimensional optical condensation of nano- and micro-particles using a gold-coated optical fibre module