In the world of bizarre but possibly useful inventions, the microboat is one of those that truly stands alone. Inventor Cheng Luo, an engineer with the University of Texas at Arlington, developed the idea following a childhood fascination with putting drops of oil on the rear of toy boats – this caused them to be propelled forward by the lower surface tension off the rear bow.
This novel effect known as Marangoni Effect has propelled Luo's unique research. Luo, currently an associate professor in mechanical and aerospace engineering, is working with researchers Hao Li, a PhD student, and post-doctoral fellow Xinchuan Liu to examine whether it might be possible to viably drive miniature boats using such an effect.
Luo describes his "eureka moment" stating, "I have a 2-year-old son. When seeing him play with toys with a big smile on his face, I often recall what I did as a kid. One day, this toy boat idea came into my mind, mostly for fun at that time. Later on, after digging deeper into this idea, I realized that, in addition to serving as toys, these small boats could also have two direct applications: material supply and sensing detection. They might be used to deliver materials to a particular location in a microchannel for chemical or biological analysis, or could be applied to carry sensors through a liquid sample for detecting toxic targets."
Luo's microfluidics research is gaining attention for its potential of serious uses in medicine and industry. However, Luo is finding that the hardest part is just building the miniature boats, which typical measure a few millimeters or less. To date the researchers are still not satisfied with the microfabrication techniques they have examined, as they involve complicated systems and structures, which would lead to high costs in eventual commercial applications.
Still the method seems promising if a viable construction process can be developed. In initial studies, Luo found that the microboats could travel at up to 30 cm per second; very good mobility for objects of its scale.
Luo states, "This propulsion method makes good use of the fore-and-aft surface tensions difference. It only needs the addition of fuel to the reservoir of a microboat. The microboat moves on its own. As such, this method eliminates the use of any complicated systems, such as propellers, rudders, anchors, and their control systems, which are normally required in macroboats. Furthermore, it does not need any external devices to provide piezoelectric, thermal, electrostatic, or magnetostatic forces, which are commonly employed in microfluidic applications to drive fluids."
The initial study involved a 19.5-mg-microboat made out of two layers of SU-8 polymers. The SU-8 polymer was selected as it is slightly denser than water, and thus relies on surface tension to float. The researchers used UV lithography to carve a nozzle and reservoir into the top layer. Instead of oil the team elected to use isopropyl alcohol as the "fuel" as it had a similar effect on surface tension. It also is thought to cause less contamination than oils, making it ideal for a variety of scenarios.
The finished boat's tiny reservoir only holds 1.49-microliters. However, it can propel the boat quite fast. The boat can travel faster in shallower bodies of water, while travelling slows in deeper bodies of water. The boat took 1.0 second to travel 30 cm in a 1.0-mm depth and 1.83 seconds to travel the same length in a pool with a 9.4-mm depth. The deeper channel initially slows the microboat by increased water resistance and then slows it even more by diluting the fuel.
The boat seems to hold even greater speed promise in longer channels. In a longer version of the 9.4-mm deep channel, the boat travelled 91.4 cm in 5.33 seconds, slightly faster on average than the short channel speed. A long shallow channel was not yet tested due to construction issues.
What are the uses of the pint-sized vessels? For one, they can help shed insight into mechanics of large boats, yielding advances in full-scale craft. Says Luo, "The motions of macroboats in narrow canals and near seaports have some common points [with microboats]: the boats are moving in shallow water or near banks. Due to the concern of experimental cost, the behaviors of large-scale ships are often explored in research labs by testing model ships, which are tens or hundreds of times smaller than their real counterparts. I am thinking of using these microboats to play some roles of the model ships. The reason behind this is simple. They are much smaller than those model ships, and thus are quite cost-effective to build and test."
Finally, Luo is dreaming big and envision his microboats leading to microsubmarines capable of traversing human blood vessels and carrying tiny cargoes. Says Luo, "Finally, in the long run the successful development of these microboats should form a foundation for developing micro ‘submarines,’ and further exploring implantable micro ‘submarines,’ which would be capable of traveling in blood vessels for active drug delivery, and disease diagnosis and treatment."