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 on whether they can be resolved individually or not. The characteristic times are measured in a wide range of wavevectors and the results are found to be in good agreement with the theoretically expected values for Brownian motion in a viscous medium. 1Microscopy and light scattering are widely used in physics, chemistry, biology and medical laboratories to access information on the structure and dynamics of mesoscopic systems.While microscopy gives direct access to real space images, scattering techniques work in reciprocal space, where information on the structure and dynamics of the system is obtained respectively from the angular and time dependence of the scattered light intensity [1]. These two complementary techniques have in general very different experimental requirements.White light sources are usual choices in microscopy, while a certain degree of coherence of the illuminating beam is required in scattering experiments; this is usually achieved by using a laser. In the past many attempts have been made to build a scattering apparatus based on a microscope; all of these attempts involved the use of a laser as an illumination source (see for example Refs. [2-6] and references therein). In some cases special care was taken to ensure the capability to perform microscopy and scattering experiments simultaneously, thereby allowing for a powerful combination of the complementary information obtained by both techniques [2,3]. More recently, microscope-based laser Dynamic Light Scattering (DLS) experiments have been developed to study the dynamic properties of samples of biological interest, such as living macrophage and red blood cells [5,6]. In practice, due to the intrinsic difficulties in building such instruments, the use of such techniques has been restricted to those laboratories, where a sufficient expertise in the realization of optical instrumentation was at hand.In this letter, we present a conceptual scheme to interpret and analyze microscopy images that are obtained from samples containing moving entities. This technique, which we term Differential Dynamic Microscopy (DDM), does not entail any special experimental requirements, being based on the use of a standard light microscope with a normal illumination source and a digital video camera. By using the tools of Fourier Optics [7] we provide the means to access information about the sample dynamics that are equivalent to the one obtained in multi-angle dynamic light scattering (DLS) experiments [8]. We test DDM by analyzing time sequences of microscopy images obtained with an aqueous dispersion of colloidal particles with diameter 73 nm, well below the resolution limit ...

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 tumors for interstitial dissemination.

Colloidal particles with directional interactions are key in the realization of new colloidal materials with possibly unconventional phase behaviors. Here we exploit DNA self-assembly to produce bulk quantities of "DNA stars" with three or four sticky terminals, mimicking molecules with controlled limited valence. Solutions of such molecules exhibit a consolution curve with an upper critical point, whose temperature and concentration decrease with the valence. Upon approaching the critical point from high temperature, the intensity of the scattered light diverges with a power law, whereas the intensity time autocorrelation functions show a surprising two-step relaxation, somehow reminiscent of glassy materials. The slow relaxation time exhibits an Arrhenius behavior with no signs of criticality, demonstrating a unique scenario where the critical slowing down of the concentration fluctuations is subordinate to the large lifetime of the DNA bonds, with relevant analogies to critical dynamics in polymer solutions. The combination of equilibrium and dynamic behavior of DNA nanostars demonstrates the potential of DNA molecules in diversifying the pathways toward collective properties and selfassembled materials, beyond the range of phenomena accessible with ordinary molecular fluids.DNA nanotechnology | limited valence colloids | critical behavior I n recent years, a strong effort has been devoted to introduce a new generation of micro-and nanocolloids interacting via strongly anisotropic forces. Anisotropic interactions can simply arise from a nonspherical particle shape or from more sophisticated physical and/or chemical patterning of the particle surface (1-7). An alternative strategy to produce complex nanoparticles is to exploit the self-assembly of DNA oligomers. The rational design of the DNA sequences enables guiding the association of multiple DNA strands into a rich variety of nanosized objects, such as geometrical figures, hollow capsules, and nanomachines, as well as more complex meso-and macroscopic structures (8-13). The selectivity of DNA binding can also be exploited to control the mutual interactions between the structures (14, 15), whereas the spontaneous assembly of DNA sequences enables producing large ensembles of particles. These properties make DNA a powerful tool to explore fundamental phenomena of soft matter and statistical physics, as indicated by previous studies of liquid-crystalline ordering and phase separations in solutions of short DNA oligomers (16-18). Here we exploit DNA self-assembly to experimentally address the phase behavior of particles interacting with specific valence, strength, and selectivity.Colloidal particles with controlled valence are the next step toward the realization of new colloidal materials and phases dependent on the presence of a small number of bonds (1-7). Theoretical and numerical studies (19) predict that a solution of low-valence particles should exhibit phase coexistence-the colloidal analog of the vapor-liquid coexistence in simple liquidsbut only at v...

We describe the use of a bright-field microscope for dynamic light scattering experiments on weakly scattering samples. The method is based on collecting a time sequence of microscope images and analyzing them in the Fourier space to extract the characteristic time constants as a function of the scattering wave vector. We derive a theoretical model for microscope imaging that accounts for ͑a͒ the three-dimensional nature of the sample, ͑b͒ the arbitrary coherence properties of the light source, and ͑c͒ the effect of the finite numerical aperture of the microscope objective. The model is tested successfully against experiments performed on a colloidal dispersion of small spheres in water, by means of the recently introduced differential dynamic microscopy technique ͓R. Cerbino and V. Trappe, Phys. Rev. Lett. 100, 188102 ͑2008͔͒. Finally, we extend our model to the class of microscopy techniques that can be described by a linear space-invariant imaging of the density of the scattering centers, which includes, for example, dynamic fluorescence microscopy.