Quantifying the dynamics of particle suspensions is of widespread interest in soft matter, including in the context of aggregates with crystalline order and of bacterial colonisation. Here, we develop a new technique to learn about these dynamics directly from microscopy images. The intensity countoscope is based on the analysis of intensity correlations in virtual observation boxes of images. Through colloidal experiments, simulations, and theory, we demonstrate how this method determines equilibrium motion properties, such as self-diffusion coefficients and particle drift. By adjusting the size of the observation boxes, we can naturally probe dynamics across different length scales. This allows us to distinguish contributions of the particle shape and the point spread function (PSF) from those of the particle motion.
This method complements the broad range of existing techniques on interpreting images, such as particle tracking, sitting right in between dynamical differential microscopy (DDM), and particle number fluctuations (Countoscope). One particular advantage of the intensity countoscope is its applicability to situations where particle tracking is difficult, e.g. at high densities, with fast dynamics or with poor image resolution. Additionally, the entire analysis is performed in real space, which makes the technique computationally cheap as it does not require Fourier transformations. It also enables to find more intuitively physical meaning behind the obtained signals. This simplicity opens up broad perspectives on the study of interparticle interactions and of collective phenomena including the physics of glasses.