We demonstrate the use of an ordinary white-light microscope for the study of the $q$-dependent dynamics of colloidal dispersions. Time series of digital video images are acquired in bright field with a fast camera, and image differences are Fourier analyzed as a function of the time delay between them. This allows for the characterization of the particle dynamics independent of whether or not they can be resolved individually. The characteristic times are measured in a wide range of wave vectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium.

Dynamics of epithelial monolayers has recently been interpreted in terms of a jamming or rigidity transition. How cells control such phase transitions is, however, unknown. Here we show that RAB5A, a key endocytic protein, is sufficient to induce large-scale, coordinated motility over tens of cells, and ballistic motion in otherwise kinetically arrested monolayers. This is linked to increased traction forces and to the extension of cell protrusions, which align with local velocity. Molecularly, impairing endocytosis, macropinocytosis or increasing fluid efflux abrogates RAB5A-induced collective motility. A simple model based on mechanical junctional tension and an active cell reorientation mechanism for the velocity of self-propelled cells identifies regimes of monolayer dynamics that explain endocytic reawakening of locomotion in terms of a combination of large-scale directed migration and local unjamming. These changes in multicellular dynamics enable collectives to migrate under physical constraints and may be exploited by tumours for interstitial dissemination.

Significance: At atomic and molecular levels, particles interact via the immutable laws of quantum mechanics, whereas at nanoscopic and microscopic levels, interactions between colloidal particles can be determined by human design. We exploit DNA self-assembly to produce nanoscopic particles with a controlled number of interacting terminations, providing the particles with valence. Experimental investigation of the collective behavior of such colloids shows that reducing the number of interacting sites results in a significant shrinkage of the gas–liquid coexistence region, with critical parameters decreasing as the valence is reduced. We also observe an unconventional dynamic behavior in the proximity of the critical point. Such findings are relevant to answer fundamental physics issues and potentially to determine the stability region of new DNA-based materials.

We present a new method to analyze quantitatively the wave front of a partially coherent x-ray beam. The technique is based on the use of two-dimensional speckle patterns combined with digital image correlation algorithms and offers a pixel size resolution, a high accuracy, and a reduced sensitivity to mechanical vibrations thanks to a very simple setup. The requirements on transverse and longitudinal coherence are also low. Finally, we show how the method can be used for phase contrast imaging applications by a single sample exposure process.