Using Shape Diversity on the Way to Structure-Function Designs for Magnetic Micropropellers

Abstract

Synthetic microswimmers mimicking biological movements at the microscale have been developed in recent years. Actuating helical magnetic materials with a homogeneous rotating magnetic field is one of the most widespread techniques for propulsion at the microscale, partly because the actuation strategy revolves around a simple linear relationship between the actuating field frequency and the propeller velocity. However, the full control of the swimmers' motion has remained a challenge. Increasing the controllability of micropropellers is crucial to achieve complex actuation schemes that in turn are directly relevant for numerous applications. The simplicity of the linear relationship though limits the possibilities and flexibilities of swarm control. Using a pool of randomly-shaped magnetic microswimmers, we show that the complexity of shape can advantageously be translated into enhanced control. In particular, directional reversal of sorted micropropellers is controlled by the frequency of the actuating field.2 This directionality change is linked to the balance between magnetic and hydrodynamic forces.We further show an example how this behavior can experimentally lead to simple and effective sorting of individual swimmers from a group. The ability of these propellers to reverse swimming direction solely by frequency increases the control possibilities and is an example for propeller designs, where the complexity needed for many applications is embedded directly in the propeller geometry rather than external factors such as actuation sequences. I. IntroductionMicroswimmers are envisioned for a multitude of applications ranging from solving environmental problems to being used for micro surgery [1][2][3]. Precise, versatile and noninvasive controllability is necessary to cover this broad scope of applications. These requirements are mostly matched by magnetic microswimmers. The fuel-free actuation by weak and homogeneous magnetic fields indeed allows remote controlling in many environments, the synthesis via nanofabrication makes them accessible even on a sub-micrometer scale [4][5][6]. In addition, the ability to functionalize their surface and the limited toxicity of the mostly iron-based propellers makes them appealing for medical applications [2,7]. Many of the current magnetic microswimmers use a helical shape with a fixed magnetic moment to rotate in an externally applied magnetic field, which enables stable propulsion. In this case, a simple linear relationship between the frequency of the actuating magnetic field and the velocity of micropropellers is used to precisely control the propeller [5,[8][9][10]. This leaves the sign of the swimming direction of the propeller to be determined by the rotation direction of the applied magnetic field, which limits the versatility of their actuation capability: when controlling two or more geometrically identical propellers, it is not possible to let them swim in a common propulsion mode respectively in the same direction and, if needed, in opposite directions, simpl...

Bead-Based Hydrodynamic Simulations of Rigid Magnetic Micropropellers

“…Thus, we conclude that, for this geometry, the direction of the magnetization should be known with an accuracy of about 5 • or better. FIGURE 6 | Velocity-frequency curve for a random-shaped propeller: (A) Experimental data (Bachmann et al, 2018); (B-D) Simulated velocity-frequency curve for different bead representations (shown in the insets): (B) 55 beads with radius a = 0.177µm; (C) 518 beads with radius a = 0.0903µm; and (D) 4276 beads with radius a = 0.04425µm. The magnetic moment direction is indicated as a blue line in the inset of (B), and was calculated as reported in section 2.5.…”

Roads to Smart Artificial Microswimmers

“…[177][178][179][180] The nonlinear speed-frequency relationship of magnetic swimmers above the step-out frequency can also be exploited for individual steering in multi-microrobot system. [181,182] In addition, magnetic field gradients, [183] specialized substrates, [184] and shape diversity [185] offer additional strategies to control multiple microrobots. Sorting of a large number of microswimmers based on size, [186] chirality, [187] and magnetic properties, [188] has been experimentally realized, contributing basic building blocks toward full control of microswimmers' motion.…”

Bidirectional Propulsion of Arc‐Shaped Microswimmers Driven by Precessing Magnetic Fields

“…by either changing frequency, [23,26,34] predefined magnetic anisotropy, [24,25,35] or complex path-planning methods. [36,37] Based on the findings of different switching points, θ t-I and θ t-II in Figure 4c, we exploit field precession for the multiagent control of the two types of swimmers (Movie S5, Supporting Information).…”