Inertial and viscous flywheel sensing of nanoparticles

TL;DR

This paper introduces a novel inertial sensing modality using rotational dynamics in microfluidics, enabling simultaneous measurement of particle volume and mass by exploiting a viscosity-driven hydrodynamic coupling.

Contribution

It demonstrates how rotational dynamics can be harnessed for inertial mass sensing, revealing particle density through fluid viscosity effects, which is a new approach in the field.

Findings

The modality measures particle volume independently of density.

Particle density is revealed when fluid viscosity dominates inertia.

The method allows high-throughput simultaneous measurement of volume and mass.

Abstract

Rotational dynamics often challenge physical intuition while enabling unique realizations, from the rotor of a gyroscope that maintains its orientation regardless of the outer gimbals, to a tennis racket that rotates around its handle when tossed face-up in the air. In the context of inertial mass sensing, which can measure mass with atomic precision, rotational dynamics are normally considered a complication hindering measurement interpretation. Here, we exploit the rotational dynamics of a microfluidic device to develop a new modality in inertial resonant sensing. Combining theory with experiments, we show that this modality normally measures the volume of the particle while being insensitive to its density. Paradoxically, particle density only emerges when fluid viscosity becomes dominant over inertia. We explain this paradox via a viscosity-driven, hydrodynamic coupling between the fluid and the particle that activates the rotational inertia of the particle, converting it into a viscous flywheel. This modality now enables the simultaneous measurement of particle volume and mass in fluid, using a single, high-throughput measurement.