Abstract
Differential dynamic microscopy (DDM) is a technique that exploits optical microscopy to obtain local, multi-scale quantitative information about dynamic samples, in most cases without user intervention. It is proving extremely useful in understanding dynamics in liquid suspensions, soft materials, cells, and tissues. In DDM, image sequences are analyzed via a combination of image differences and spatial Fourier transforms to obtain information equivalent to that obtained by means of light scattering techniques. Compared to light scattering, DDM offers obvious advantages, principally (a) simplicity of the setup; (b) possibility of removing static contributions along the optical path; (c) power of simultaneous different microscopy contrast mechanisms; and (d) flexibility of choosing an analysis region, analogous to a scattering volume. For many questions, DDM has also advantages compared to segmentation/tracking approaches and to correlation techniques like particle image velocimetry. The very straightforward DDM approach, originally demonstrated with bright field microscopy of aqueous colloids, has lately been used to probe a variety of other complex fluids and biological systems with many different imaging methods, including dark-field, differential interference contrast, wide-field, light-sheet, and confocal microscopy. The number of adopting groups is rapidly increasing and so are the applications. Here, we briefly recall the working principles of DDM, we highlight its advantages and limitations, we outline recent experimental breakthroughs, and we provide a perspective on future challenges and directions. DDM can become a standard primary tool in every laboratory equipped with a microscope, at the very least as a first bias-free automated evaluation of the dynamics in a system.
Highlights
For about 50 years, since the development of affordable laboratory laser sources, it has been possible to perform dynamic light scattering (DLS) experiments
In Differential dynamic microscopy (DDM), image sequences are analyzed via a combination of image differences and spatial Fourier transforms to obtain information equivalent to that obtained by means of light scattering techniques
We can expect in just a few years to be able to run DDM in real time on video feeds of over 100 fps; this would be the ultimate application of DDM avoiding even the need for the storage of large quantities of data, reducing the image flow to its intrinsic dynamical information content, providing real-time physical measurement, and allowing experimental design based on advanced imaging-driven triggers
Summary
For about 50 years, since the development of affordable laboratory laser sources, it has been possible to perform dynamic light scattering (DLS) experiments. Implementing DDM does not require advanced high-end optics; consumer-led demand for its two underlying technologies (computing and digital cameras) is rapidly propelling it forwards in terms of speed and power; with small variations, DDM has been shown to extract a whole set of parameters typical of the traditional approaches outlined above, and more This perspective aims to capture the research made possible by DDM in its first decade, including mostly very recent developments, and to suggest untapped potential areas where it could be fruitful. To make a very concrete example from living systems, one wants to isolate intracellular motions from the motions of cells themselves, and even if focusing on cell motion, it is often the case that the character of motion changes across a time or length scale, as persistence is lost (e.g., in run and tumble bacterial motility dynamics) These are typical scenarios where the DDM approach is ideal
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