Optimization of comb-driven devices for mechanical testing of polymeric nanofibers subjected to large deformations

Comb-driven electrostatic actuators applied to mechanical testing of nanostructures are usually designed by a "brute-force"approach for maximum electrostatic-force output, which results in limited actuation range. This issue is more prevalent when testing soft nanofibers with large ductility. In this paper, the design considerations for a comb-driven platform for nanoscale mechanical testing of ductile nanofibers subjected to 50%, or larger, inelastic extensions are presented. The optimization carried out aimed at increasing the net-force output by comb drives with clamped-clamped tethers, which also improves on the accuracy in the calculation of the force that is applied onto the nanofiber specimens. At large actuator motions, tethers of low bending stiffness increased the net force applied to a nanofiber and provided better accuracy in the calculation of the applied force. On the contrary, at small actuator motions, the maximum net-force output by the comb drives increased with the axial tether stiffness due to the associated increase in the pull-in-instability voltage. The fabricated surface-micromachined devices enabled experiments with individual electrospun polyacrylonitrile nanofibers at a maximum force of 30 μN and extensions up to 60%. The force output calculated from the voltage input to the electrostatic devices was compared to direct measurements by an independent optical method.