Nanofabricated and microfabricated devices, ranging from nanomotors propelled by ultrasound to microelectromechanical systems (MEMS), can move in fascinating ways through liquids and over solid surfaces. The motion of these devices gives them the ability to interact with their environments, enabling a variety of emerging applications in healthcare and electronics. However, the dynamics of nanoscale and microscale devices are often characterized by motions which are too small or fast to capture using existing measurement methods. Measuring such dynamics can be essential to understanding the performance and reliability of the devices. In some such cases, nanoparticles can be used as optical indicators of the motion of the devices. The goal of this project is to provide facility users with access to optical microscopes and measurement methods to image and track nanoparticles indicating device motion. We develop innovative measurements, while taking care to understand measurement interactions between nanoparticles and devices and to quantify measurement uncertainties. Our recent studies have resulted in surprising new insights into the dynamics of small devices, including the fastest reported, kilohertz rotation of a nanomotor in water, and the previously unknown, highly erratic nanoscale motion of a standard MEMS.
Acoustic Nanomotor Dynamics
Nanorods propelled by ultrasound are an emerging nanotechnology with potential applications in healthcare. In contrast to chemically, magnetically, or electrically actuated nanomotors, acoustically actuated nanomotors can transport through liquid phase environments with high ionic strengths and without toxic fuels or external magnetic or electric fields. To fulfill the potential of nanomotors propelled by ultrasound for rapidly transporting through and strongly interacting with biological media, the basic dynamics of the nanomotors must be clearly understood.
Such nanomotors, typically metallic rods, exhibit several modes of motion in the presence of acoustic actuation. Both translation along and rotation around the principal longitudinal axis of single nanomotors have been observed, as well as collective motions involving many nanomotors. Rotation is a particularly interesting mode of motion that motivates further study, for several reasons. First, hydrodynamic interactions of nanomotors with other objects or one another are determined in part by the vortical flows around nanomotors, which relate to the rates of nanorod rotation. Second, any correlation, or lack thereof, between rotation and translation is important for understanding, engineering, and applying these modes of motion. Third, rotation is difficult to directly image for nanomotors that are rapidly rotating and optically featureless around the longitudinal axis, so that rates of rotation are quantitatively unknown and potentially very high. For these reasons, measuring rotational motion is essential to understanding and using this dynamic nanotechnology.
In this project, we use nanoparticles as optical indicators of the microvortical flows around gold nanorods propelled by ultrasound in an acoustic resonator. We track the microvortex advection of nanoparticle tracers around rotating and translating nanomotors by darkfield localization microscopy. We input our measurements of this motion into a hydrodynamic model to infer the rotational frequencies of the nanomotors. We find that nanomotors rotate at frequencies of up to ≈2.5 kHz, or ≈150,000 RPM, establishing the fastest reported rotation of a nanomotor in water. Our measurements emphasize the importance of precision nanofabrication in future measurements and applications of nanomotors. We also find that rotation and translation are independent and variable modes of motion within experimental uncertainty. These surprising results are essential to understanding the behavior of nanomotors propelled by ultrasound, and this unprecedented combination of small size and fast rotation is highly relevant to emerging biomedical applications of nanomotors, as well as other applications involving nanoscale transport, mixing, machining, assembly, and rheology.
Nanoscale mechanical motion
As mechanical devices are scaled down to microscale and nanoscale dimensions, imaging the motion of the devices becomes more challenging. The devices themselves have very small dimensions, and their characteristic motions can be many orders of magnitude smaller. Displacements and rotations of small mechanical devices can be particularly difficult to measure in the case that the plane of the motion is parallel to the plane of the surface over which the device is operated. Such is the case for many types of linear and rotary sensors and actuators. Commercial instruments for imaging in-plane motion do not achieve the requisite precision, while emerging methods for measuring in-plane motion with high precision often require specific test structures or specialized measurement systems. Therefore, advancing the development of widely applicable methods to image the motion of small mechanical devices is a promising approach towards a better and broader understanding of device performance.
In this project, we label a small mechanical device with a microscale constellation of nanoparticles used as optical indicators. By localizing each nanoparticle in the constellation, we can measure the position and orientation of the device under test, and thus the kinematics of the system, with high precision. In a proof of principle study, we track the motion of a scratch drive electrostatic actuator over a surface. We measure displacements and rotations with uncertainties of < 2 nm and < 100 μrad, respectively, enabling the observation of single stepwise motions, which have not been previously observed. We find that the motion of the scratch drive is characterized by local, transient variations in nanoscale step size, which are not predicted by existing models of the behavior of this standard device. These surprising results show that our method can have broad impact in related studies of MEMS motion, facilitating the fabrication and application of MEMS. We are further developing our measurement methods and applying them to study the motion of other MEMS.