Optical lever for broadband detection of fluid interface fluctuations

TL;DR

This paper presents a compact optical lever setup capable of detecting minute fluctuations of a liquid-air interface across a broad frequency range, enabling detailed analysis of fluid interface dynamics.

Contribution

The study introduces a novel optical lever method with broad frequency detection for fluid interfaces, bridging optical techniques with soft-matter system analysis.

Findings

Detects interface fluctuations over five orders of magnitude in frequency.

Capable of measuring low-frequency eigenmodes and high-frequency capillary waves.

Demonstrates broad applicability and practical utility in fluid dynamics.

Abstract

We exploit the optical lever principle to detect minute fluctuations of a liquid-air interface. Waves propagating on the interface deflect a specularly reflected laser beam, inducing angular deviations captured by a dual-element photodiode. We implement this principle in a compact set-up that includes a temperature-controlled fluid sample. This allows us to detect deflection angle fluctuations across five orders of magnitude in frequency, from individual low-frequency surface eigenmodes to the thermal distribution of high-frequency capillary waves. In addition to demonstrating the method's versatility and broad dynamical range, we highlight practical considerations in characterising liquid interface dynamics, bridging established optical methods with their application to fluid and soft-matter systems.