Magnetotactic bacteria possess organelles called magnetosomes that confer a magnetic moment on the cells, resulting in their partial alignment with external magnetic fields. Here we show that analysis of the trajectories of cells exposed to an external magnetic field can be used to measure the average magnetic dipole moment of a cell population in at least five different ways. We apply this analysis to movies of Magnetospirillum magneticum AMB-1 cells, and compare the values of the magnetic moment obtained in this way to that obtained by direct measurements of magnetosome dimension from electron micrographs. We find that methods relying on the viscous relaxation of the cell orientation give results comparable to that obtained by magnetosome measurements, whereas methods relying on statistical mechanics assumptions give systematically lower values of the magnetic moment. Since the observed distribution of magnetic moments in the population is not sufficient to explain this discrepancy, our results suggest that non-thermal random noise is present in the system, implying that a magnetotactic bacterial population should not be considered as similar to a paramagnetic material.
Disordered nanostructures with correlations on the scale of visible wavelengths can show angle-independent structural colors. These materials could replace dyes in some applications because the color is tunable and resists photobleaching. However, designing nanostructures with a prescribed color is difficult, especially when the application—cosmetics or displays, for example—requires specific component materials. A general approach to solving this constrained design problem is modeling and optimization: Using a model that predicts the color of a given system, one optimizes the model parameters under constraints to achieve a target color. For this approach to work, the model must make accurate predictions, which is challenging because disordered nanostructures have multiple scattering. To address this challenge, we develop a Monte Carlo model that simulates multiple scattering of light in disordered arrangements of spherical particles or voids. The model produces quantitative agreement with measurements when we account for roughness on the surface of the film, particle polydispersity, and wavelength-dependent absorption in the components. Unlike discrete numerical simulations, our model is parameterized in terms of experimental variables, simplifying the connection between simulation and fabrication. To demonstrate this approach, we reproduce the color of the male mountain bluebird (Sialia currucoides) in an experimental system, using prescribed components and a microstructure that is easy to fabricate. Finally, we use the model to find the limits of angle-independent structural colors for a given system. These results enable an engineering design approach to structural color for many different applications.
A holographic microscope captures interference patterns, or holograms, that encode three-dimensional (3D) information about the object being viewed. Computation is essential to extracting that 3D information. By wrapping low-level scattering codes and taking advantage of Python's data analysis ecosystem, HoloPy makes it easy for experimentalists to use modern, sophisticated inference methods to analyze holograms. The resulting data can be used to understand how small particles or microorganisms move and interact. The project illustrates how computational tools can enable experimental methods and new experiments.
Abstract. We introduce a novel technique to produce monodisperse droplets through the snap-off mechanism. The methodology is simple, versatile, and requires no specialized or expensive components. The droplets produced have polydispersity < 1% and can be as small as 2.5 µm radius. A convenient feature is that the droplet size is constant over a 100-fold change in flow rate, while at higher flows the droplet size can be continuously adjusted.Microfluidics applications often require emulsions with a wide range of characteristics, prompting the development of several distinct techniques for producing droplets [1,2]. One important parameter is the degree of polydispersity among droplet sizes, where smaller values are preferable for many applications. Droplets with an extremely low polydispersity are particularly desirable for basic science investigations of emulsions [3,4], vessels for tiny experiments [5,6,7], as well as calibration in both academic and industrial settings [8]. Here we present a method we have recently developed using glass capillaries and a surface tension driven 'snap-off' instability to produce droplets. This method is remarkable for its simplicity, ease of implementation, and the high monodispersity of droplets produced. An additional convenience is that there are two distinct regimes of droplet production: 1) the size of droplets is insensitive at low flow rates; while, 2) at high flow rates the droplet size is tunable.The snap-off instability of droplets in cylindrically symmetric capillaries was first described in 1970 [9], and has since been investigated further in the context of understanding the physics behind snap-off [10,11]. We took advantage of this effect to develop a versatile system for production of monodisperse droplets that is easy to assemble and operate. One important consideration is that this setup requires no flow of the continuous phase, since the pinch-off is driven by surface tension forces rather than viscous forces. Although the snap-off process has been used previously to produce droplets in flattened microfluidic geometries [12,13,14], our simple cylindrical configuration is able to produce droplets that are more monodisperse.In order to prepare monodisperse droplets we have utilised a method that is schematically depicted in Fig.
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