Optically driven Archimedes micro-screws for micropump applications: multiple blade design

Abstract

ABSTRACT We study the rotation of photo-driven Archimedes screw with multiple blades. The micron-sized Archimedes screws are readily made by the two-photon polymerization technique. Free-floating screws that are trapped by optical tweezers align in the laser irradiation direction, and rotate spontaneous ly. In this study we demonstrate that the rotation speeds of two-blade-screws is twice the rotation sp eed of one-blade-screw. However, more complex 3-blade-screws rotate slower than 2-blade-screws due to their limited geometry resolution at this micron scale. Keywords: Archimedes screw, multiple blades, two-photon polymerization, optical tweezers, optical torque, micropump 1. INTRODUCTION Two-photon polymerization (TPP) is an attractive method for the fabrication of three-dimensional (3D) microstructures. The chemical polymerization reaction occurs at the laser focus where photo-initiator chromophores are excited by the simultaneous absorption of two photons. Three-dimensional micrometer-size objects with sub-micron resolutions can easily be produced. These micro-objects can easily be trapped, and displaced by optical forces that are generated by highly focused laser beams [1]. In addition, particles with anisotropic shapes can rotate around the laser axis due to unsymmetrical forces that are generated at their surfaces. Su ch remotely photo-driven micr o/nanorotors are unique tools to elaborate mechanical and se nsing functions at the micron and nano scales . They require no mechanical contact and no electrical wire. By means of this technique, some photo-driven micro-rotors have been proposed in recent years. Photo-driven rotations can be induced either from the net optical torque resulting from the complex shape [2,3] or by driving a movable part with an optical tweezers trap [4]. Recent work s have focused on the realiza tion of more complex devices that integrate the optical motor with its laser power [5], an d on the demonstration of optical driven micropumps in their microfluidic channels [6-8]. Previously, we have demonstrated the rotation of photo-driven Archimedes screws [9]. The screw rotates spontaneously around its long axis when it is trapped at the laser focal point. The laser-induced rotation is due to the optical torque that is transferred by the laser scattering on the screw. The rotational speed is linearly proportional to the optical power. We found that micro-screws with shorter pitche s rotate slower than micro-sc rews with longer ones. Their optofluidic properties were quantitatively described by the conservation of momentum that occurs when the laser photons are reflected on the helical micro-screw surface [10]. We also demonstrated the photo-driven micropump action in a microfluidic channel with a non-optimized flow rate of 6pL/min. In this work, we have studied the influence on the number of blades on the rotation speeds of photo-driven Archimedes micro-screws. As expected by the macroscopic experience, the rotation speed of two-blade-screws is twice the rotation speed of one-blade-screw. However, the 3-blade-screws, that are more difficult do fa bricate due to the limited resolution, rotate slower than 2-blade-screws.