When blood suspension penetrates a capillary radius by wetting, the advancing meniscus decelerates rapidly when the blood cell volume fraction is above a certain critical concentration. Below the critical concentration, blood suspension behaves like a homogeneous liquid and the wetted length increases as the 0.5 power of time. We attribute the former deceleration dynamics to a unique packing mechanism behind the meniscus that is driven by radial migration of the deformable blood cells. Unlike rigid particle suspensions, a concentrated slug develops behind the meniscus of blood suspension and its concentration increases linearly with respect to the meniscus position downstream due to this packing mechanism. As the suspension viscosity blow up with a −2 power with respect to blood concentration φ at maximum packing, viscous dissipation at the slug quickly controls the meniscus speed if the slug length is comparable to the total wetted length, thus significantly delaying the meniscus penetration dynamics. The critical concentration is measured empirically and shown to be a linear function of the capillary radius R with a simple scaling theory. For 40% whole blood, penetration rate is too slow, in the order of μm /s at 2cm from the entrance, to be widely used in sample loading for miniature diagnostic kits with diameter less than 26 micron.

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