How small can a propeller be?
-Ambarish Ghosh, Electrical Communication Engineering
Helical propulsion is at the heart of locomotion strategy utilized by various microorganisms in fluidic media. The effects of thermal fluctuations in these systems can give rise to many fascinating features, both in isolation, as well as through fluidic interactions in a collection of internally powered (self propelled) swimmers. In recent years, significant efforts have been directed towards the construction of artificial helical swimmers (propellers) which are maneuvered by rotating magnetic fields. These so called helical propellers not only form an interesting soft matter system, but also have tremendous potential to be useful in various biomedical applications, such as drug delivery, intra-cellular microrheological measurements etc.
Recently, we reported the first study of velocity fluctuations of an artificial helical propeller. Unlike self propelled swimmers, the motion of artificial propellers is tied to the characteristics of the external power source, and therefore their fluctuations show different behavior than the internally powered swimmers. Through experiments and numerical simulations, we study the speed fluctuations and the directionality of a propeller as a function of different experimental parameters.
The most important question asked here is how small an artificial propeller can be, for which it was important to look at the microscopic mechanisms that “caused” the speed fluctuations in this system with multiple degrees of freedom. Through numerical simulations, as well as through Granger causality analysis, we show an overwhelming importance of orienational fluctuations as the size of the propellers was reduced. Most interestingly, we find that below the length scale of around a micron (considering aqueous media); irrespective of the available power, the operational frequency of the propulsion system needs to go up as inverse of the cube of their size. These observations raise questions on how small can a practical propeller be. While the smallest propeller made till date is about 1.5 microns, it may not be easy to reduce this size limit much further than this in aqueous media, thus eluding many promising nanoscale applications envisioned with “nano”-propellers.
'Velocity Fluctuations in Helical Propulsion: How Small Can a Propeller Be'; Arijit Ghosh, Debadrita Paria, Govindan Rangarajan and Ambarish Ghosh; Journal of Physical Chemistry Letters, 2014, 5, 62-68.