![]() Table 1. Properties of the samples S1 to S5: weight fraction of glycerol in the solvent, refractive index n solv of the solvent from tables, count-rate CR measured with DWS RheoLab, obtained l* based on CR, viscosity η measured with DWS RheoLab at a temperature of 20☌. Thus, an increase in the glycerol concentration yields an increased l*. In our system, an increase in glycerol concentration leads to an increase in n solv which decreases the optical contrast of the tracer particles. Furthermore, l* is dependent on the refractive index of the solvent n solv (see Table 1). In the case of dilute systems l* is inversely proportional to the tracer particle concentration. The cornerstone of accurate DWS measurements is ensuring the applicability of the diffusion approximation which requires that L≥ 5 l*, where L is the thickness of the cuvette. The choice of the tracer particle concentration is connected to the cuvette size. Table 1 summarizes the different samples S1 to S5. We used a tracer particle concentration of approximately 2 wt% (solid content) for all samples. Tracer particles, consisting of polystyrene beads with a diameter of 980 nm dispersed in water, were added. Mixtures with different ratios of glycerol and water were prepared. It is the most important optical parameter for turbid samples. Accurate values of l* are necessary not only for extracting viscoelastic information from DWS measurements but also for many other applications. The smaller l* is, the stronger the scattering. It thus indicates how turbid a sample is. This is the distance after which a scattered photon has totally lost its original propagation direction. Viscosity of glycerol free#Moreover, we explain how one can measure the transport mean free path l* over an extended range of turbidity. As a model system, we use mixtures of glycerol with water at different mixing ratios and compare the measured viscosities with tabulated (published) values. The purpose of this application note is to show how one can prepare a transparent sample by adding tracer particles to obtain sufficient turbidity for DWS measurements. However, if the sample is transparent, some sample preparation is required prior to measurement. The DWS RheoLab from LS Instruments is a versatile tool to conduct such measurements.īecause DWS requires sufficient turbidity to ensure strong multiple scattering of laser light, samples such as creams, milk, yoghurt, lotions, shampoos, paints, and ceramics are well suited. The method is based on the analysis of the fluctuations of laser light that is scattered multiple times within a sample. The video below shows several different long chained oils, each progressively more viscous.Diffusing Wave Spectroscopy (DWS) is a powerful optical technique primarily used to study the rheological properties of turbid samples. Glycerol, CH 2OHCHOHCH 2OH, is viscous partly because of the length of the chain but also because of the extensive possibilities for hydrogen bonding between the molecules. Fuel oil, lubricating grease, and other long-chain alkane molecules are quite viscous for this reason. This is because the molecular chains get tangled up in each other like spaghetti-in order for the liquid to flow, the molecules must first unravel. Liquids containing long molecules are invariably very viscous. Honey, mostly glucose and fructose (see image below) is a good example of a liquid which owes its viscosity to hydrogen bonding. Liquids whose molecules are polar or can form hydrogen bonds are usually more viscous than similar nonpolar substances. Viscosity is governed by the strength of intermolecular forces and especially by the shapes of the molecules of a liquid. Those like ether or gasoline which flow very readily have low viscosities. Liquids which flow very slowly, like glycerin or honey, have high viscosities. The resistance to such flow is called the viscosity. \)īecause its molecules can slide around each other, a liquid has the ability to flow. ![]()
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