Interpreting the power spectral density of a fluctuating colloidal current

Area of Science:

• Physics, Soft Matter
• Physical Chemistry
• Nanotechnology

Background:

• Molecular transport through nanopores involves complex stochastic processes causing current fluctuations.
• Understanding these noise mechanisms is crucial but experimentally challenging due to molecular system complexity and inaccessibility.
• Existing models often simplify the interplay between particle dynamics and transport noise.

Purpose of the Study:

• To construct a controllable colloidal model system for studying particle transport fluctuations.
• To experimentally investigate the origins of current noise in confined systems.
• To establish quantitative links between particle dynamics, system geometry, and noise characteristics.

Main Methods:

• Development of a microfluidic system using hard spheres as model particles.
• Experimental measurement of particle currents and their fluctuations.
• Analysis of power spectral density (PSD) and particle velocity distributions.
• Comparison with theoretical models for shot noise and finite transit time.

Main Results:

• Particle currents exhibit fluctuations due to random arrival times and variable particle speeds.
• Characteristic scalings in the PSD were observed and rationalized by a finite transit time shot noise model.
• Particle velocity distributions were found to reflect confining geometry and fluid flow profiles.
• Details of velocity distributions were shown to govern the PSD, linking noise to underlying mechanisms.

Conclusions:

• The colloidal model system provides a tractable platform for studying transport fluctuations.
• This work establishes concrete links between PSD characteristics and underlying physical mechanisms in particle transport.
• Findings pave the way for a systematic understanding of noise in driven systems, including biological and synthetic nanopores.