Autonomous Motion of Metallic Microrods Propelled by Ultrasound

Abstract

Autonomously moving micro-objects, or micromotors, have attracted the attention of the scientific community over the past decade, but the incompatibility of phoretic motors with solutions of high ionic strength and the use of toxic fuels have limited their applications in biologically relevant media. In this letter we demonstrate that ultrasonic standing waves in the MHz frequency range can levitate, propel, rotate, align, and assemble metallic microrods (2 μm long and 330 nm diameter) in water as well as in solutions of high ionic strength. Metallic rods levitated to the midpoint plane of a cylindrical cell when the ultrasonic frequency was tuned to create a vertical standing wave. Fast axial motion of metallic microrods at ~200 μm/s was observed at the resonant frequency using continuous or pulsed ultrasound. Segmented metal rods (AuRu or AuPt) were propelled unidirectionally with one end (Ru or Pt, respectively) consistently forward. A self-acoustophoresis mechanism based on the shape asymmetry of the metallic rods is proposed to explain this axial propulsion. Metallic rods also aligned and self-assembled into long spinning chains, which in the case of bimetallic rods had a head-to-tail alternating structure. These chains formed ring or streak patterns in the levitation plane. The diameter or distance between streaks was roughly half the wavelength of the ultrasonic excitation. The ultrasonically driven movement of metallic rods was insensitive to the addition of salt to the solution, opening the possibility of driving and controlling metallic micromotors in biologically relevant media using ultrasound.

Reversible Swarming and Separation of Self-Propelled Chemically Powered Nanomotors under Acoustic Fields

“…[1][2][3] The cell was made in a steel plate (50 mm x 50 mm x 0.94 mm) with a 240 μm Kapton tape as a protective layer, and a sample reservoir at the center (1 mm), covered by a 18 mm x 18 mm x 0.15 mm glass slide for ultrasound reflection. The piezoelectric transducer which produces the ultrasound waves (Ferroperm PZ26 disk 10 mm diameter, 0.5 mm thickness) was attached to the bottom center of the slide by conductive epoxy glue.…”

Ultrasound-Modulated Bubble Propulsion of Chemically Powered Microengines

“…4,5 The cell was made in a covered glass slide (75 x 25 x 1 mm). The piezoelectric transducer which produces the ultrasound waves (Physik Instrumente PZT ring 0.5 mm thickness, 10 mm outside diameter by 5mm center hole diameter) was attached to the bottom center of the glass slide.…”

Observation of Metal Nanoparticles for Acoustic Manipulation

“…As the nanoparticles in the experiments were not treated with a surfactant to prevent the self‐assembly, the actual manipulation effect may depend on the size of agglomerates rather than that of single nanoparticle. Recently, different metal nanorods have also been explored for acoustic manipulation, exhibiting interesting behaviors in ultrasonic response, including levitation, chain assembly, reversible swarming, axial propulsion, rolling, ring patterns, streak patterns, and the others 42, 43, 44. Although the nanorods in these experiments showed nanosized structures on tangent plane, the dominated parameter for the nanoscale acoustic manipulation should be further discussed to exclude the influence of their microscale length.…”

“…Acoustic Experiments : The acoustic manipulation system was constructed as previously described with small modifications 42, 43, 44. Briefly, the cylindrical reservoir with a height of 150 µm and a circular hole of 5 mm diameter cut in the center was created by attaching three layers of polyimide Kapton tapes on a piece of polished silicon wafer (76.2 mm × 76.2 mm × 381 µm).…”