Department of Electrical Engineering and Automation

Tractor beam and microfluidics

Research | Robotic Instruments

main / research / wetting and microfluidics

Laser beams have long been imagined as being able to pull objects toward their sources. Such phenomena, initially introduced in the movie Star Wars and referred to as tractor beams, have been observed later at microscopic scales. However, all previous work has been limited to solid micro- and nanoparticles, ranging from hundreds of nanometers to tens of micrometers. 

We show that a tractor beam can work on large-sized objects: a laser tractor beam can pulla millimeter-sized water droplet on a superhydrophobic surface towards the light source through a two-stage photothermal mechanism: advances due to contact angle hysteresis and coalescence with condensation satellite droplets, followed by iterative bubble bursting, coalescence, and relaxation - a combination of effects not reported earlier. 

Laser tractor beam for water droplet

We have designed methods to control liquid spreading using easy-to-fabricate silicon undercut structures, which can confine the wetting of liquids even with very low surface tension. We have also discovered that the electron beam of the environmental scanning electron microscope (ESEM) can create precise patterns that have extreme wetting contrast of 150º and features as small as 1 µm. We also applied the hydrophilic-superhydrophobic patterns for gravity-induced rapid deposition of nano-liter droplets and sliding deposition. 

Cover of Advanced Materials journal magazine for year 2013, Vol. 25, No. 16. The background is gray. On center there is a shperical droplet dyed blue, lying on the surface of silicon. Around it there are two perfectly circular rings of water. The reflection on the Silicon surface cast their mirror image.
Undercut structure controlling droplet spread.
robotic instruments - Maskless, High‐Precision, Persistent, and Extreme Wetting‐Contrast Patterning in an Environmental Scanning Electron Microscope
E-beam writing of superhydrophilic wetting patterns on superhydrophobic substrate.
robotic instruments - Sliding droplets on hydrophilic/superhydrophobic patterned surfaces for liquid deposition
Sliding distributing of nanoliter droplet on patterned substrate.

Selected publications:

  1. Han, P., Zhu, Z., Cenev, Z.M., Liimatainen, V., Zhang, H., Chang, B. and Zhou, Q., "Laser-Induced Photothermal Pulling of Dyed Droplets on a Superhydrophobic Surface", Langmuir, 41(18), pp.11374-11381, 2025.
  2. Liimatainen, V., Sariola, V., Zhou, Q., "Controlling liquid spreading using microfabricated undercut edges", Advanced Materials, Vol. 25(16), pp. 2275–2278 (frontispiece on page 2274), 2013.
  3. Liimatainen, V., Shah. A., Johansson, L., Houbenov, N. and Zhou, Q., “Maskless, high-precision, persistent and extreme wetting-contrast patterning in an environmental scanning electron microscope”, Small, vol. 12 (14), pp. 1847-1853, 2016.
  4. Chang, B., Kivinen, O., Pini, I, Levkin, P.A., Ras, R.H.A., Zhou, Q., “Nanoliter Deposition on Star-shaped Hydrophilic-superhydrophobic Patterned Surfaces”, Soft Matter, 4 (36): 7500-7506, 2018.

  5. Chang, B., Zhou, Q., Ras, R., Shah, A., Wu, Z., Hjort, K., “Sliding droplets on hydrophilic/superhydrophobic patterned surfaces for liquid deposition”, Applied Physics Letters, vol. 108, 154102, 2016.

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