Brownian, or random thermal, motion acts on slender objects (like bacteria, sperm cells, flagella, nanotubes and nanowires) when they are suspended in a liquid thanks to random collisions between the liquid molecules. In most real-world situations, the objects are confined by boundaries, such as substrate surfaces and biological cell membranes. Quantifying so-called hydrodynamic interactions with these boundaries is thus crucial – not only for better understanding how biological objects behave but also for improving nanoparticle assembly during nanoelectronic device fabrication.

A team led by Xiang Zhang of the University of California at Berkeley and the Lawrence Berkeley National Laboratory says that it has now precisely observed the Brownian motion of nanowires tethered on a substrate in three dimensions. The researchers say they have also understood the effect of complex hydrodynamic interactions between the substrate and the nanowires thanks to their 3D “string-of-beads” computer model.

Zhang and colleagues obtained their results using a dynamic interference imaging technique that allowed them to observe how the tethered nanowires moved in a liquid. In their experiments, the researchers suspended silicon nanowires that were 150 nm wide and between 5 and 25 µm long in a closed chamber on a glass slide immersed in water. The ends of the nanowires were attached to the glass surface thanks to van der Waals forces. The other ends were free and underwent Brownian rotation around the tethered point.

When the team exposed their samples to monochromatic light, the light that was back scattered from the nanowires interfered with that reflected from the water-glass interface, so producing a periodic interference pattern along the wires. “By simple image processing, this single-interference image allows us to measure both inclined and azimuth angles of the tethered nanowires at the same time – something that enables us to track how the nanowires rotate in solution,” team member Sadao Ota told nanotechweb.org.

Powerful tool

“Our study allows us to better understand how slender particles move near walls and barriers,” he added. “The novel interferometery technique, together with our computer model, will be a powerful tool for analysing interfacial microrheology of various soft condensed matter materials, including biological particles near cellular membranes.”

The work will also be important for understanding nanomaterial self-assembly on substrates for when it comes to making nanoelectronic devices. “Indeed, our string-of-beads model could even help to optimize the experimental conditions during assembly and potentially reduce position inaccuracies caused by thermal fluctuations and hydrodynamic forces,” said Ota.

The team says that it will now be studying ferromagnetic nanowires using its interferometery technique. “And since our numerical calculation is so efficient and versatile, we hope to use it to simulate more complex and practical situations such as directed assembly of nanowires within top-down fabricated electrical devices,” explained Ota.

The current work is detailed in Phys. Rev. E 89 053010.