Instrumentation Journal of Physics 35 No. 2 (1989) 274-281 Concentric Cylinder Viscometer Extremely Low Shear Stress. R. T. Rodriguez, Raul Montiel C. and A. Romo U. Department of Physics, University Autonomy Metropolitan - Iztapalapa, PO Box 55-534, 09340 Mexico, DF (Received August 2, 1988; accepted January 13, 1989)Summary.This paper describes the design and construction of a viscometer of Zimm-Crothers type, which allows very accurate viscosity measurements at various valuesof shear stressesusing only an extremely small rotor.AbstractWe describe the design and construction of a Zimm Crothers type viscosimeter whichpermits us to get very accurate neasurements of viscosity at several values of theextremely low shear stress, using only one rotor. 1. Introduction directly proportional to viscosity.One of the oldest techniques and used It may happen, however, that thein the characterization of materials in viscosity of the polymer solution issolution is plyometrics viscometry. This dependent on the flow conditions of theis based on the fact that the presence instrument used: in these cases it isof large particles dissolved or said that the non-Newtoniansuspended in a liquid produced a viscosity. This type of behavior is foundradical change in the flow property mainly in solutions of highly asymmetricof the system. molecules rigid or flexible molecules in solutions of very high molecularA very important advantage of this weight.technique is that the amount isdetermined experimentally, either The capillary viscometer has proven toviscous or the torque flow time is be a versatile and inexpensive,
however, due to progress in the studyof biological macromolecules, has beenfound that many of these molecules aredegraded under the action of evensmall shear forces found in the FIGURE 1. The diagram shows theviscometers capillaries. Furthermore, position of the rotor in thesome biological and synthetic stator.molecules have non-Newtonianbehavior when subjected to shear 2. Design and constructionstresses experienced in regular sizedcapillaries. The viscometer of Zimm-Crothers type , is a concentric cylinder viscometer, which is made entirelyIn view of this, there is need for a of glass, this feature provides theviscosity comparable to convenience facility to work with almost anyand availability of a capillary solvent.viscometer, but operating at high shear The outer cylinder (called the stator)rates of several orders of magnitude remains fixed, while the inner lower than the capillary. cylinder (called the rotor) is rotating (Fig.1).The instrument described herein ischeap, puts the solution in contact with At the bottom of the stator wasonly glass, runs at different shear placed a tube for introducing thestresses and has been used in cutting sample is from the bottom of theforces from 0.003 to 0.0008 dynes/cm2, viscometer. This device alsowhich are several orders of magnitude facilitate filling, very accuratelyless than commonly, used capillary adjusts the height of the rotor on the viscometers. stator core to be reproducible in the viscosity measurements, in general, the relative viscosity measurements have an accuracy of no more than 0.2% . All the viscometer is introduced into a heating jacket (Fig. 2) which allows the device to operate at different temperatures. As temperature control was used brand Haake recirculating bath with platinum resistance control, which
controls with accuracy of ±0.05°C.The viscometer should be mountedrigidly to preserve the geometry ofthe system. For this is supportedwith a nylon ring which is mountedon a bracket that allows fivemovements: first, the position of theviscosity on the external magneticfield is carried out through threeplatforms that move in the x - y -z; second, the orientation thereofwith respect to the magnet takesplace by three screws placed in thenylon ring which allow movementzenithal and azimuthal (θ, φ) (Fig. FIGURE 2. The figure shows a2). It is important to note that the cross section of the concentricalignment of the viscometer with cylinder viscometer and therespect to the external magnetic description of its components.field is vital to prevent movement ofprecession of the rotor. where P is the period of revolution for the solution to the solvent P0 andBy determining the viscosity is Pm to the external magnetic field. Indirectly proportional to the time of our case having us that Pm = 0.1revolution of the rotor as shown in second, thus a negligible amount isthe following expression: about P which is on the order of 300 seconds and P0 is the order of 90 − seconds. = − To determine this period of revolution is made a small mark on the aluminum which is observed using a cathetometer, which is fixed to the worktable. Was used additionally an electronic counting, which operates in the following manner: a disk mounted with regularly spaced perforations on the
synchronous motor, and through an supported solely by flotation, can be optocoupler mounted on the base of used the viscometer (Fig. 2), the frequency was measured angle of only a single rotor with liquids rotation of the disc which resulted to whose densities vary in a range of be of 16.4-Hz, the signal obtained less than five percent. Fig 5 shows by rotating the disc is sent to an a schematic diagram of the electronic counter, the reading of experimental equipment and Figs. 6 this is proportional to the viscosity of and 7 show graphs of the calibration the solution in the viscometer. In Fig curves of viscosity for toluene and 3 shows a photograph of the carbon tetrachloride, respectively. In instrument, and in Figs. 4a and 4b Tables I, II, III summarizes some of show schematic diagrams of the physical characteristics of the electronics involved in this device. instrument. It is very important to keep a careful There have been several attempts cleaning of the rotor in operation, it by other authors [2, 3], in order to should not be touched with fingers automate this instrument by adding while the viscometer is placed in, as optical devices, which allow more this will cause problems in flotation precise measurements. and centered.Figure 3. The photograph shows theviscometer mounted on its base.It should be noted that the density ofthe liquid is very important in the use ofthis viscometer, because the rotor is
3. Experimental results. tetrachloride (CCl4 PBIC on. Molecules of poly-(butylisocyanate) As mentioned above, some of the for not too high molecular weights systems suitable for use in this type (less than 105) have the form rigid viscometers are those in which the rod. molecules are asymmetrical, or polymer molecules which have the Due to the asymmetrical shape of form of rigid rods, this is because the molecules, they have a phase the analysis of viscometer these transition, which was predicted by solutions must be made extremely Flory  of an isotropic state in low cutting speed. which all the molecules have random orientations to a nematic Because of this, we used this type state in which there is a viscometer on solutions of poly- direction privileged along which the (butylisocyanate) in carbon polymer chains tend to align. This phase transition modifies the viscosity of the solution, and is intended to detect this by viscometric measurements. However, the viscosity must Perforated disc output FIGURE 4a. The photo detector output signal is a square type of 0-5V.FIGURE 4b. Electronics concentric cylinder viscometer.
Have very low shear for the velocity field does not induce the phase transition. Two solutions were prepared in CCl4 PBIC. Was analyzed each of these solutions at different temperatures in the range of 18°C to 42°C. In Figure 8a shows a graph of experimental results obtained for the system in CCI4 PBIC. Clearly shows that there is a discontinuity in the viscosity when the temperature changes. This graph was made at a concentration of 8.6 x 10-4g / g. In Figure 8b shows the detail of the transition for the same system at a concentration of 9x10-4g/g. FIGURE 5. The figure shows theexperimental setup used. FIGURE 6. Calibration curve of toluene.FIGURE 7. Calibration curve for carbon tetrachloride. FIGURE 8A. Log graph of viscosity vs. reciprocal temperature for the solution-CCI4 PBIC the concentration of 8.6x10-4g/g.
FIGURE 8. Log graph of viscosity vs. reciprocal temperature for the solution-CCI4 PBlC the concentration 9.0x10-4g/g.TABLE 1. Physical data of the stator and TABLE 2. Details of the aluminumrotor comprising the concentric cylinder core of the rotor. viscometer.TABLE 3. Comparative data between the concentric cylinder viscometer and acapillary viscometer typical.4. ConclusionsThe concentric cylinder viscometer presented here, despite being a little more difficultthan using a capillary viscometer, practically does not disturb the system under study,the shear so small that they can be obtained with this instrument at all, in some cases ,the only means which can measure the viscosity of solutions or suspensions of largeparticles.
5. References: 1. H. H. Zimm & C.M.Crothers, Proc. Nat. Acad. Sci. 48 (1962) 905. 2. W. H. J. Stork & H. Vroome, J. Phys. E: Sci lnst 5 (1972) 314. 3. H. J. Seherr, H. C. Vautine & L.P. Witnaver, J. Phys E: Sci Inst 3 (1970) 322. 4. P. J. Flory, Proc. Roy. Soc. London A234 (1956) 73.