Optical diffraction tomography (ODT) using Born or Rytov approximation suffers from severe distortions in reconstructed refractive index (RI) tomograms when multiple scattering occurs or the scattering signals are strong. These effects are usually seen as a significant impediment to the application of ODT because multiple scattering is directly linked to an unknown object itself rather than a surrounding medium, and a strong scatter invalidates the underlying assumptions of the Born and Rytov approximations. The focus of this article is to demonstrate for the first time that multiple scattering and high material contrast, if handled aptly, can significantly improve the image quality of the ODT thanks to multiple scattering inside a sample. Experimental verification using various phantom and biological cells substantiates that we not only revealed the structures that were not observable using the conventional approaches but also resolved the long-standing problem of missing cones in the ODT.

Microfluidic mixing plays a key role in various fields, including biomedicine and chemical engineering. To date, although various approaches for imaging microfluidic mixing have been proposed, they provide only quantitative imaging capability and require exogenous labeling agents. Quantitative phase imaging techniques, however, circumvent these problems and offer label-free quantitative information about concentration maps of microfluidic mixing. We present the quantitative phase imaging of microfluidic mixing in various types of polydimethylsiloxane microfluidic channels with different geometries; the feasibility of the present method was validated by comparing it with the results obtained by theoretical calculation based on Fick's law.

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Microfluidic mixing plays a key role in various fields, including biomedicine and chemical engineering. To date, although various approaches for imaging microfluidic mixing have been proposed, they provide only quantitative imaging capability and require for exogenous labeling agents. Quantitative phase imaging techniques, however, circumvent these problems and offer label-free quantitative information about concentration maps of microfluidic mixing. We present the quantitative phase imaging of microfluidic mixing in various types of PDMS microfluidic channels with different geometries; the feasibility of the present method was validated by comparing it with the results obtained by theoretical calculation based on Fick's law. IntroductionMicrofluidic devices that control micrometre-sized samples in fluids are essential tools in various research fields such as analytical chemistry, biology and medicine [1,2]. Mixing in microfluidic channels has great potential due to the capability of controlling droplet volume, chemical concentration, and sorting of fluids [3]. Recently, microfluidic mixing has been used for pharmaceutical developments and medical diagnostics [4,5].In order to monitor microfluidic mixing, various optical imaging techniques have been proposed and used [6]. For example, phase contrast microscopy, differential interference contrast microscopy, and confocal microscopy have been employed to observe the mixing and extract information of fluid mixture. Among various methods, bright-field and fluorescence microscopy are well-established and readily available [7]. However, existing techniques have limitations in providing quantitative imaging information. Most fluids are transparent and thus do not provide enough imaging contrast. Although the use of exogenous fluorescent or dye molecules can enhance imaging contrast, these techniques provide only qualitative information on the local concentrations of fluids and also raise the issues of altering intrinsic fluid conditions. This imaging constraint is unfortunate because when microfluidic mixing can be quantified, microfluidic devices have much to offer with their unique control capabilities, high flexibility, and avoidance of the use of large amounts of samples. Despite the many challenges, there is a strong motivation to provide quantitative imaging of microfluidic mixing, especially if that imaging can be achieved without introducing exogenous labeling agents. Refractive index (RI) variation in microfluidic mixing can be detected by various optical methods including schlieren microscopy, speckle photography, and surface plasmon resonance [8][9][10][11][12][13][14]. However, these techniques do not provide quantitative measurements of microfluidic concentration.To overcome the limitations in the imaging of microfluidic mixing, quantitative phase imaging (QPI) techniques can be employed. QPI is based on interferometric microscopy; this process quantitatively and precisely measures the optical phase delay maps introduced by a sample. Recently, QPI has been...

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