Magnetically Responsive Polymer Nanopillars With Nickel Cap

Magnetically Responsive Polymer Nanopillars with Nickel Cap

Zhiren Luo^*1, Xu Zhang², and Chih-Hao Chang¹

¹Walker Department of Mechanical Engineering, University of Texas at Austin

Austin, TX 78712, USA

²Department of Mechanical and Aerospace Engineering, North Carolina State University

Raleigh, NC 27695, USA

Magnetically responsive micro-/nanoscale pillars is a promising method to implement active structures, and have attracted research interests for their advantages such as low energy consumption, non-contact actuation, and fast response. A common strategy of embedding magnetic particles within polymer matrix is a facile method to fabricate such structures, however the balance between mechanical compliance and magnetic susceptibility cannot be decoupled.[1-4] Here we demonstrate a new type of magnetically responsive nanostructure consisting of a polydimethylsiloxane (PDMS) nanopillar array with nickel caps deposited on the tops and sidewalls. This nanostructure successfully decouples the compliance and susceptibility by concentrating magnetic material onto the pillar surface and away from the base.

During the fabrication processes, two types (type I and type II) of geometries have been designed to characterize the significance of several factors such as magnetic torque component, aspect ratio, duty cycle. First, the anti-reflection coating with 90 nm thickness and the photoresist with 2.7  and 1.3  thickness for type I and II, respectively, are spincoated on silicon substrates. Then the photoresist is exposed twice orthogonally using Lloyd’s mirror interference lithography with 325 nm laser, resulting in two-dimensional (2D) periodic holes array with periods of 2  and 0.8  for type I and II, respectively. Then PDMS is applied into the photoresist mold, cured, and demolded mechanically, leading to replica of pillar array. The type-I PDMS pillar is trimmed to form narrow pillar base and high aspect ratio using reactive ion etching. The nickel cap is then deposited onto the type-I pillar. In comparison, nickel cap and sidewall are directly deposited onto the type-II PDMS pillars to characterize the magnetic torque component and duty cycle.

Magnetic and mechanical models based on magnetic force and torque have been developed for two types of nanopillars. It demonstrates that the magnetic torque component can significantly improve the actuation. The model was used to study the weakening of the actuation by the residual magnetic layer and fabrication methods to mitigate its effect. It indicates that a high duty cycle can reduce the unit area of residual layer on the base and therefore improve the actuation as well. The actuation result of type-II nanopillars has been analyzed using image processing, leading to a maximum tilting angle of about 8 which matches with the analytical model. This structure demonstrates a feasible strategy for magnetic actuation at such scale for various potential applications, such as tunable optical elements and cell manipulation.

KEYWORDS: magnetic responsive, nanopillar, weakening effect, hierarchical structure, field gradient

*zhiren.luo@utexas.edu

[1] Drotlef, D.-M. et al. Adv. Mater. 26, 775-779 (2013).

[2] Peng, Y. et al. Adv. Funct. Mater. 25, 5967–5971 (2015).

[3] Evans, B. A. et al. J. Magn. Magn. Mater. 324, 501–507 (2012).

[4] Luo, Z. et al. ACS Appl. Mater. Interfaces 12, 11135-11143 (2020)