Tracer-free Contactless Acoustic Microrheometry Quantifies Viscoelastic Spectrum of Phase-separated Condensates

TL;DR

This paper introduces a novel tracer-free, contactless acoustic microrheometry technique to measure the viscoelastic properties of microscale phase-separated condensates across a range of frequencies, overcoming limitations of existing methods.

Contribution

The authors develop and validate a versatile acoustic platform for quantitative, non-invasive viscoelastic characterization of condensates at single-microscale resolution.

Findings

Successfully measures size- and frequency-dependent mechanical responses of dextran condensates.

Reveals salt-dependent internal viscoelastic changes in nucleic-acid condensates.

Provides a broadly applicable framework for studying phase-separated materials.

Abstract

The rheology of phase-separated condensates plays a central role in applications spanning advanced materials design and cellular processes, yet quantitative characterization of their viscoelasticity remains challenging due to the limitations of existing microrheological methods that require tracer particles or mechanical contact. Here, we establish tracer-free and contactless acoustic microrheometry as a versatile platform for quantifying the frequency-dependent complex shear modulus of single microscale condensates over 0.01-10 Hz. Using spatiotemporally controlled acoustic radiation force generated within a micro-acoustic resonator, this method deforms condensates for creep-recovery and oscillatory viscoelastic measurements. Quantitative validation using dextran condensates in a polyethylene-glycol continuous phase successfully captures their size- and frequency-dependent mechanical responses, while application to nucleic-acid condensates reveals salt-dependent internal viscoelastic changes at single-condensate resolution. By enabling quantitative dissection of condensate mechanics without invasive probes, acoustic microrheometry provides a broadly applicable framework for investigating phase-separated condensates across materials science, soft matter physics, biology, and beyond.