Determination of Rheologic Properties

Choice of Viscometer:

To accurately understand and assess how a particular substance flows, it's crucial to select the appropriate measuring tool. For instance, in materials where shear rate (how quickly layers of the substance move past each other) is directly related to the force applied, instruments that work at a fixed shear rate can be used.

These instruments give us one point on a graph showing how the substance flows. By extending a line from this point back to where the flow began, we can get a complete picture of how the substance behaves when force is applied. However, this method assumes that the substance behaves like water, which is called being "Newtonian."

Unfortunately, not all substances behave like water. If a substance is "non-Newtonian," meaning its flow characteristics change under different conditions, using a single-point instrument is not enough to understand its behaviour. Therefore, it's important to use instruments that can measure the substance's flow at different speeds.

These multipoint instruments can provide a full picture of how non-Newtonian substances behave. For example, they can show how the viscosity of a substance changes when it's at rest (with no force applied) compared to when it's being stirred, poured, or spread (with moderate to high force applied). Single-point instruments can't capture these changes accurately.

Capillary Viscometer:

To find out how thick a Newtonian liquid is, we can use a capillary viscometer. It's like a small capillary where the liquid flows down because of gravity.

We measure how long it takes for the liquid to flow between two marks on the tube. A newer version of this tool is shown in Figure 19-11.

We compare the time it takes for the unknown liquid to flow with the time it takes for a liquid of known thickness, usually water, to flow between the same marks.

If we call the viscosity of the unknown liquid η1 and the known one η2, and their densities ρ1 and ρ2, and their flow times t1 and t2 in seconds, we can use these values in an equation to figure out the exact viscosity of the unknown liquid, η1.

Falling-Sphere Viscometer:

In this type of viscometer, a ball, usually made of glass or steel, moves down a nearly vertical tube filled with the liquid we're testing. The tube is kept at a constant temperature. How fast the ball falls depends on how thick the liquid is.

The Hoeppler viscometer, which you can see in Figure 19-13, is a device that works on this principle. We put the liquid and the ball into the tube and let them reach the same temperature as the surrounding water. Then, we turn the tube upside down, so the ball starts at the top.

We carefully measure how long it takes for the ball to fall between two marks on the tube, and we do this a few times to get an accurate result. We can then use these measurements to calculate the Viscosity of the liquid.

"To find out how quickly a ball falls in a liquid, you need to know the time it takes (t) in seconds. Sb and Sf are specific gravity of the ball and the liquid are at the temperature we're working with. Each type of ball has its own number (B) that helps us calculate. The manufacturer gives us this number. We can use this method with different sizes of glass and steel balls and a wide range of liquid thickness, from 0.5 to 200,000 poise. It's best to use a ball that takes at least 30 seconds to fall for accurate results."

Cup-and-Bob viscometers :

In cup-and-bob viscometers, the liquid being tested gets sheared between the outer edge of a spinning bob and the inner side of a cup that holds the bob. You can see how this works in Figure 19-14. Different viscometers work in slightly different ways. Some rely on rotating the cup, while others rotate the bob. In the Couette type, it's the cup that rotates. As the liquid rubs against the bob, it makes the bob spin. The strength of this spin, or torque, tells us how thick or thin the liquid is.

 Stromer Viscometer :

The Stormer viscometer instrument is a commonly used viscometer, which operates based on the Searle principle. This instrument, which is a modification of one described by Fischer, is illustrated in Figure 19-16. Here's how it works:

1. Setting up: The liquid being tested is placed between a cup and a bob in the viscometer, and the system is allowed to reach the same temperature.
2. Measuring revolutions: A weight is hung on a hanger, and the bob starts rotating. The time taken for the bob to complete 100 revolutions is noted.
3. Converting data: The number of revolutions is then converted into revolutions per minute (rpm). The weight on the hanger is then increased, and the process is repeated. This helps create a rheogram, which is a graph of rpm against the weight added.
4. Converting to shear rates: With the help of certain constants, the rpm values can be converted into shear rates, which are measured in seconds to the power of -1.
5. Converting to shear stress: Similarly, the weights added can be converted into units of shear stress, which are measured in dynes per square centimeter.
6. Limitations: According to Araujo, the Stormer instrument shouldn't be used with liquids that have a viscosity below 20 centipoise (cp).

For this type of viscometer, the equation changes slightly. The rotational speed, represented by Ω (omega), is determined by the torque, represented by T, in dynes per centimeter. The depth to which the bob is immersed in the liquid is denoted by h, while Rb and Rc stand for the radii of the bob and the cup, respectively. You can refer to Figure 19-14 for a visual representation of these components.

 Cone-and-Plate Viscometer :

The Cone-and-Plate Viscometer, such as the Ferranti-Shirley viscometer, is a type of device used to measure viscosity. Here's a breakdown of how it works:

1.     Setup: The viscometer consists of a plate and a cone, as shown in Figure 19-18. The sample is placed at the center of the plate, which is then positioned under the cone.

2.     Shearing the Sample: A motor controls the rotation of the cone. As it spins, the sample is sheared in the narrow gap between the stationary plate and the rotating cone.

3.     Adjusting Shear Rate: The speed of rotation, measured in revolutions per minute (rpm), can be increased or decreased using a selector dial. The viscous traction or torque, which represents the shearing stress, is displayed on an indicator scale.

4.     Data Collection: By varying the rotational speed and observing the torque readings, a graph of rpm or shear rate against torque or shearing stress can be created.

5.     Calculating Viscosity: For Newtonian liquids, the viscosity in poise can be calculated using an equation involving an instrumental constant (C), the torque reading (T), and the speed of the cone (v) in rpm.

6.     Handling Non-Newtonian Liquids: For materials exhibiting plastic flow, the plastic viscosity and yield value can be determined. The plastic viscosity is calculated using a specific equation, while the yield value is derived from the torque at the shearing stress axis (Tf) and another instrumental constant (Cf).

In simpler terms, this viscometer measures how easily a liquid flows. It does this by spinning a cone in the liquid and measuring how much force it takes to spin. This information helps us understand the liquid's thickness.