The balls we tested were ordered from Ball Supply Corporation (Avon, CT) in two sizes, d = 0.508 mm (or 0.0200 in.) and d = 0.4 mm, both of Grade 25, which specifies the uncertainty in diameter to be within 25 × 10 −6 in. Tiny stainless steel balls, which are commercially available as ball bearings in various sizes with high precision. A small volume of a few milliliters suffices for dozens of experiments. ![]() Test liquids, such as mixtures of water and glycerol in various mass ratios. The publication of this study is timely in light of recent explosion of research at the micro- or nano-scales, often involving small amounts of soft, aqueous, or petroleum materials. We believe this systematic effort helps to solidify the basis for a broader range of applications using this simple technique. The focus of this study is on a simple design for measuring small samples, confirming that shear viscosity of common Newtonian fluids can be reliably determined through proper calibration. 3,10–12 However, we found no previous effort to rigorously calibrate the measured apparent viscosity against standard viscous fluids in order to reliably determine the dynamic viscosity of those biological fluids. Biomedical researchers have actually used such a device in order to measure and compare viscosity of fluids available in only tiny volumes. The measurements can be made within seconds and are readily repeatable. Here we describe a miniature ball-drop device for measuring fluid viscosity in sample volumes as small as ∼100 μl. 8 By extrapolating measurements to zero characteristic stress, the ball drop technique has been shown to yield zero-shear-rate viscosity of polymer solutions. The ball drop device has also been applied to measure dynamic viscosity of non-Newtonian fluids, by varying the size and density of the balls used. By recording the progressively slower flow rate into the capillary due to the increasing viscous drag, the device has been shown to yield the dynamic viscosity of non-Newtonian fluids as a function of shear rate. 7 The test fluid infiltrates into a long capillary in the chip, driven by the capillary force. Recently, a microliter viscometer has been developed, using disposable, microfluidic chips with long channels. Viscometers based on capillary flow may yield shear dependent viscosity if the driving stress is systematically varied, although most commercial devices are not designed to do so. 2,3 A newer technique known as particle tracking microrheology, 4–6 both active and passive, yields dynamic viscosity by tracking the motion of micro-sized beads embedded in the fluids of interest. Commercial rheometers are designed to perform a range of measurements while varying either the shear rate by a constant rotation or the frequency of an oscillatory shear strain. Non-Newtonian fluids, for which the dynamic viscosity varies with shear rate, pose challenges for most existing methods.
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