Non-Buoyant Microrobots Swimming with Near-Zero Angle of Attack

Abstract

In the design of microrobots, a helical geometry is pivotal to overcome the time-reversal constraints of the scallop theorem. The helical geometry enables the microrobots to propel themselves forward in viscous fluids with a corkscrew like motion when they are allowed to rotate. It is physically advantageous for microrobots to swim with near-zero angle of attack much like buoyant microorganisms, allowing high thrust for forward propulsion. This type of propulsion is not possible as the non-buoyant microrobot drifts downward due to gravity. Here, we analyze the stability problem of controlling magnetically driven helical microrobots to achieve bounded straight runs without drift in a low-Reynolds-number regime. We demonstrate periodic active suspension solutions, that facilitate helical propulsion with minimal angle of attack and zero drift. We theoretically predict unique control inputs, for a given helical microrobot geometry and magnetic composition (i.e., 62% Ni and 24% Au Wt%), which can be generated with rotating field and field-gradient pulling. Using microrobots fabricated of denser-than-water soft-magnetic body (4870 kg•m<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>), we find that the microrobot is allowed to swim with near-zero angle of attack of 8.3° ±5.2° (mean ±s.d.), outperforming conventional gravity compensation methods.