 Award ID(s):
 2104837
 NSFPAR ID:
 10511600
 Publisher / Repository:
 American Physical Society
 Date Published:
 Journal Name:
 Physical Review E
 Volume:
 108
 Issue:
 3
 ISSN:
 24700045
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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The trajectory of sperm in the presence of background flow is of utmost importance for the success of fertilization, as sperm encounter background flow of different magnitude and direction on their way to the egg. Here, we have studied the effect of an unbounded simple shear flow as well as a Poiseuille flow on the sperm trajectory. In the presence of a simple shear flow, the sperm moves on an elliptical trajectory in the reference frame advecting with the local background flow. The length of the majoraxis of this elliptical trajectory decreases with the shear rate. The flexibility of the flagellum and consequently the length of the major axis of the elliptical trajectories increases with the sperm number. The sperm number is a dimensionless number representing the ratio of viscous force to elastic force. The sperm moves downstream or upstream depending on the strength of background Poiseuille flow. In contrast to the simple shear flow, the sperm also moves toward the centerline in a Poiseuille flow. Far away from the centerline, the crossstream migration velocity of the sperm increases as the transverse distance of the sperm from the centerline decreases. Close to the centerline, on the other hand, the crossstream migration velocity decreases as the sperm further approaches the center. The crossstream migration velocity of the sperm also increases with the sperm number.more » « less

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Abstract Threedimensional dynamics of flexible fibers in shear flow are studied numerically, with a qualitative comparison to experiments. Initially, the fibers are straight, with different orientations with respect to the flow. By changing the rotation speed of a shear rheometer, we change the ratio
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null (Ed.)An inline hologram of a colloidal sphere can be analyzed with the Lorenz–Mie theory of light scattering to measure the sphere's threedimensional position with nanometerscale precision while also measuring its diameter and refractive index with partperthousand precision. Applying the same technique to aspherical or inhomogeneous particles yields measurements of the position, diameter and refractive index of an effective sphere that represents an average over the particle's geometry and composition. This effectivesphere interpretation has been applied successfully to porous, dimpled and coated spheres, as well as to fractal clusters of nanoparticles, all of whose inhomogeneities appear on length scales smaller than the wavelength of light. Here, we combine numerical and experimental studies to investigate effectivesphere characterization of symmetric dimers of micrometerscale spheres, a class of aspherical objects that appear commonly in realworld dispersions. Our studies demonstrate that the effectivesphere interpretation usefully distinguishes small colloidal clusters in holographic characterization studies of monodisperse colloidal spheres. The effectivesphere estimate for a dimer's axial position closely follows the ground truth for its center of mass. Trends in the effectivesphere diameter and refractive index, furthermore, can be used to measure a dimer's threedimensional orientation. When applied to colloidal dimers transported in a Poiseuille flow, the estimated orientation distribution is consistent with expectations for Brownian particles undergoing Jeffery orbits.more » « less