Quantitative optical microscopy of colloids: The legacy of Jean Perrin

Abstract

When the discontinuous structure of matter was yet an intriguing hypothesis, Jean Perrin performed a set of elegant and pioneering experiments that marked the birth of what today we consider quantitative optical microscopy. Picking up the baton from Perrin, today microscopists face incredible challenges, aiming to extract quantitative information from the increasingly content-rich and complex images made available by modern microscopy techniques. Here, I provide an overview of these challenges and describe the solutions adopted to succeed in this complex task when investigating colloidal systems or systems in which colloidal particles are embedded as microrheological probes.

Tracking-Free Determination of Single-Cell Displacements and Division Rates in Confluent Monolayers

“…Equations (9,10) are valid under the assumption that the only relevant degree of freedom contributing the optical signal is the one associated with the displacement of the centroids of the particles. In particular, the particles should maintain their shape and orientation over time.…”

“…Apparently, our procedure, by making use of Equation (10), relies of the relatively strong assumption that cellular displacements follow a Gaussian probability distribution function (PDF).…”

“…Being DDM equivalent to a scattering technique [8,9], the multi-scale information extracted from it is in general revealing of the collective effects on the system. For instance, DDM performed on Brownian colloidal particles mostly probes their collective dynamics rather than the self one, which is usually the result of real-space particle tracking approaches [10]. However, while for inert colloidal particles a theoretical framework exists [11] that enables to extract a large amount of information about the particle-particle interactions from the q resolved scattering information extracted by DDM [12], using a similar approach with cell monolayers is a challenging feat.…”

Relationship between rheology and structure of interpenetrating, deforming and compressing microgels

“…In this work, we propose a framework to explain the frequency-dependent linear viscoelasticity of microgel suspensions, composed of micron-sized poly(N-isopropylacrylamide) (pNIPAM) microgels with a dense core and a fuzzy corona, from weakly compressed packings to strongly overpacked states by combining the results from oscillatory shear measurements and nanoscale imaging. To this end we characterize the macroscopic rheological properties of soft particle suspensions at a constant temperature and take advantage of the advent of microscopic structural information about individual microgels and pairs of microgel particles resolved via dSTORM superresolution microscopy 41 . Our aim is to describe and connect the mechanisms that determine the viscoelastic and in particular the dissipative behavior across the different concentration regimes.…”

Control and optical mapping of mechanical transitions in polymer networks and DNA-based soft materials

“…2e) Light and X-ray scattering techniques [64,65] can access multiple length scales but are typically limited in terms of time and space resolution. Optical imaging [66] is often the most natural choice, although blind to phenomena occurring at the nanoscale. In particular, the in situ combination of rheology and microscopy [62,6] and microrheology [67,23], which measures viscoelastic properties on a local scale, can help to bridge macroscopic and microscopic behaviours.…”