title: Density and Shape Effects in the Acoustic Propulsion of Bimetallic Nanorod Motors.
abstract: Bimetallic nanorods are propelled without chemical fuels in megahertz (MHz) acoustic fields, and exhibit similar behaviors to single-metal rods, including autonomous axial propulsion and organization into spinning chains. Shape asymmetry determines the direction of axial movement of bimetallic rods when there is a small difference in density between the two metals. Movement toward the concave end of these rods is inconsistent with a scattering mechanism that we proposed earlier for acoustic propulsion, but is consistent with an acoustic streaming model developed more recently by Nadal and Lauga ( Phys. Fluids 2014 , 26 , 082001 ). Longer rods were slower at constant power, and their speed was proportional to the square of the power density, in agreement with the acoustic streaming model. The streaming model was further supported by a correlation between the disassembly of spinning chains of rods and a sharp decrease in the axial speed of autonomously moving motors within the levitation plane of the cylindrical acoustic cell. However, with bimetallic rods containing metals of different densities, a consistent polarity of motion was observed with the lighter metal end leading. Speed comparisons between single-metal rods of different densities showed that those of lower density are propelled faster. So far, these density effects are not explained in the streaming model. The directionality of bimetallic rods in acoustic fields is intriguing and offers some new possibilities for designing motors in which shape, material, and chemical asymmetry might be combined for enhanced functionality.
authors.affiliation: Departments of Chemistry, Physics, and Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Suzanne
authors.initials: S
authors.lastname: Ahmed
authors.affiliation: School of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology , Shenzhen 518055, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
authors.affiliation: School of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology , Shenzhen 518055, China.
authors.forename: Lanjun
authors.initials: L
authors.lastname: Bai
authors.affiliation: Departments of Chemistry, Physics, and Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Dillon T
authors.initials: DT
authors.lastname: Gentekos
authors.affiliation: Laboratoire de Physique et Mécanique des Milieux Hétérogènes, UMR7636 CNRS , UMPC, ESPCI, 10 rue Vauquelin, 75005 Paris, France.
authors.forename: Mauricio
authors.initials: M
authors.lastname: Hoyos
authors.affiliation: Departments of Chemistry, Physics, and Biochemistry and Molecular Biology, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Mallouk
keywords: acoustic motor; bimetallic nanomotors; nanomotor; ultrasonic propulsion