Pressure rheology describes the determination of a material’s flow or deformation behavior under increased pressure above ambient, higher than 1 bar (100 kPa). Pressure rheology is typically used for the following applications:
When pressure is applied to a liquid, the viscosity typically increases. The only exception is water below 32 °C and some water-based solutions. It is assumed that the effect is caused by the free volume of the liquid, which is reduced when pressure is applied. This leads to an increase of inner friction and therefore to an increase in viscosity.[2]
The increase of viscosity depends strongly on the material and molecule structure. Compared to temperature, however, pressure influences are significantly smaller. Many liquids with a simple molecule structure show minor viscosity changes in a moderate pressure range up to several hundred bar, and therefore, the influence of pressure can often be neglected for technical applications. Materials with longer molecules can be affected more significantly by increased pressure. Especially for branched molecules, which have additional free inner volume, the pressure effect can be increased. The pressure influence is often non-linear and increases further with increasing pressure. This is especially relevant for lubrication, where very high pressures can appear. Also, in the field of oil recovery, pressure influences can become significant.[1][2]
Figure 1 Pressure influence on the viscosity of crude oil
Figure 1 shows the pressure influence on crude oil up to 1,000 bar. It can be seen that the influence of pressure increases significantly above 600 bar.
In many cases, pressure is applied to prevent evaporation of volatile materials or drying. This way, the viscosity of materials at increased temperature can be measured to simulate the technical processing behaviour. Also, aging experiments under increased temperature can be carried out. This type of testing is commonly used in the petrochemical and food industry.
In general, pressure can be applied either by a fluid pump or by a gas. When using a non-dissolving gas or at least a gas that dissolves very little (typically N2), the results of a measurement are expected to be the same as when a liquid pump is used. If the gas dissolves to a significant extent, however, it typically reduces the viscosity of the material. In that case, the gas can act as a plastifier. This is often done in the polymer industry to replace solvents for polymer processing, e.g. for polymerization or polymer blending with CO2 gas[3].
The following application report describes how different gases can influence the viscosity of polymer melts: PDMS Measurements Under Pressure Influence of the Used Gas for Pressurization
Hydrate formation strongly depends on pressure and temperature and can be displayed in pressure-temperature diagrams. When hydrates start to form, the liquid becomes a slurry and, as a result, the viscosity increases. This is especially important for oil recovery since the appearance of hydrates can lead to increased pressure drops or even to a complete blockage in production lines. Based on rheological measurements under elevated pressure, such behaviour can be predicted and precautions can be taken.[4]
An example of such a measurement is displayed in Figure 2. It shows hydrate formation indicated by the peak in viscosity at a temperature of 5.5 °C.
Figure 2: Temperature-dependent viscosity curve of crude oil mixed with salt water at 50 bar pressure. At around 5.5 °C, hydrate formation occurs. [4]
Besides hydrate formation, crystallization may play a role at elevated pressure. For more information, please see the following application report: Rheological Measurements at High Pressures and Low Temperatures
Find additional information in the e-learning course: High-pressure rheology
Figure 3 a) Gas pressure cell
A pressure cell is a completely sealed system. The main challenge is to transduce the torque from the rheometer to the inside of the pressure cell to achieve a rotational motion of the measuring system. This is typically done via magnets.
Generally, a distinction can be made between two different types of pressure cells: gas pressure cells and liquid pressure cells.
a) Gas pressure cells
Figure 3a shows the construction principle of a gas pressure cell. The sample is indicated in blue. The pressure is applied by a gas, which is located above the sample. Depending on the application, different gases can be used. The cell is connected to a pressure cylinder or a gas pressure pump to apply the pressure. The torque is transduced via an inner and an outer magnet. The inner magnet is connected with the measuring shaft, where the measuring system is attached. The shaft is held in place by two ball bearings. When the outer magnet, which is connected to the head of the rotational rheometer, starts to rotate, the inner magnet follows and thus the torque of the instrument is transduced inside the pressure cell.
Figure 3 b) Liquid pressure cell
b) Liquid pressure cells
In a liquid pressure cell, the bearings are directly in contact with the sample (see Figure 3b). In this case, sapphire bearings are used. The inner friction of that bearing type is higher compared to ball bearings, which leads to lower sensitivity. The pressure is applied by mechanical compression of the liquid itself, using a liquid pump. The torque is transduced by magnets, in the same manner as for the gas pressure cells.