Viscosity Probes

The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal "thickness" concept: for example, syrup has a higher viscosity than water. Viscosity can be conceptualized as a quantification of the frictional forces created between adjacent fluid layers that are moving relative to each other. For example, when a fluid is forced through a tube, it flows faster near the axis of the tube than near its wall. In this case, experiments have shown that some stress (such as the pressure difference between the two ends of the tube) is needed to maintain the flow through the tube. This is because a force is needed to overcome the friction between the fluid layers in a relative motion state: the strength of this force is proportional to the viscosity. Fluids that are not resistant to shear stress are called ideal or inviscid fluids. Zero viscosity was observed only at very low temperatures in superfluids. Otherwise, the second law of thermodynamics requires that all fluids have a positive viscosity. Technically, this fluid is viscous or viscous. Fluids with a high viscosity (such as asphalt) may appear to be solid.

Figure 1. Viscosity.

Definition

In materials science and engineering, one is often interested in understanding the forces, or stresses, involved in the deformation of a material. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. These are called viscous stresses. For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared; rather, they depend on how quickly the shearing occurs. Viscosity is a material property that correlates viscous stress in a material with the rate of change (strain rate) of deformation. Although it works for general flows, it is easy to visualize and define in simple shear flows (such as flat Couette flows).

Figure 2. Illustration of a planar Couette flow. Since the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. The relative strength of this force is a measure of the fluid's viscosity.

Viscosity Probes

We offer a range of unique organic viscosity probes (molecular rotors) that are ideal for microscopy, such as spatial imaging of microviscosity within a single domain within a living cell, and many other applications in life and physical sciences. BOC Sciences's method is based on the fluorescence detection of a unique fluorescent molecule called a molecular rotor, which exhibits a strong fluorescent response to the viscosity of the surrounding environment in both lifetime and emission spectra. We provide a series of high light stability probes with different excitation regions and emission wavelengths, as well as a large dynamic sensing range (fluorescence and viscosity response) and different water solubility. These probes are designed to meet your various viscosity needs Tailored. All our probes are pre-packaged and sold dry, which can be reconstituted with the addition of only 50 µL of solvent.

Reference:

The discussion which follows draws from Chapman & Cowling 1970, pp. 232-237

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