There are many methods for measuring surface tension and interfacial tension, and it is difficult to determine which method is the most suitable measurement method. You may have heard of mechanical and optical methods, and you may have heard of the plate method and the ring method, but if the measurement results are the same, why do you have so many measurement methods? Since surface tension and interfacial tension measurements are widely used in different fields, there are significant differences between samples. When selecting a measurement method, the sample and application direction need to be considered.
In this application article, we will provide a tool for method selection. By asking questions about the sample and application direction, you can determine which method is best for the test.
Quickly view measurement methods
The most commonly used methods for measuring surface tension and interfacial tension are the ring method, the plate method, and the hanging drop method. The first two are mechanical methods, and the last one is optical method.
The mechanical method is to measure the change in force between a gas and a liquid or between a liquid and a liquid at a probe position. The probe is connected to a high precision balance and the liquid interface is in contact with the probe. The balance is calculated by the balance of the force when the probe is in contact with the liquid interface to calculate the surface tension. The force depends on factors such as the shape and size of the probe, the contact angle between the probe and the liquid, and the surface tension of the liquid. The shape and size of the probe are easy to control. The probe is usually made of platinum to ensure that the probe's contact angle with the sample is close to 0 degrees. Both probes are often used in tests: plates and rings. Platinum rods can also replace plates when the amount of sample is limited.
Surface tension and interfacial tension can also be measured using the hanging drop method in the optical method. The shape of the liquid suspended at the needle depends on the balance of forces, including the surface tension of the test liquid. Modern computing uses an iterative method to compute the solution of the Young-laplace equation. Therefore, the surface and interfacial tension of two incompatible liquid bodies of known density can be calculated.
Method selection problem
There are a few things that need to be considered in advance to choose the most appropriate measurement method for your application. Below we summarize some of the issues.
Is there a need for standardized measurement methods? The mechanical method is the most common test method for measuring surface tension in international standards, although some optical methods still exist. The list of tensile measurement standards is shown in Table 1. If your application needs to comply with industry standards, then you need to choose a test method based on the criteria.
Whether the sample volume is limited In some applications, especially biological samples, the sample volume is very limited due to the expensive or difficult to obtain samples. In this case, a small sample size measurement method must be selected.
In the mechanical method, the required sample amount is in the milliliter range. The exact amount of sample is determined by selecting the sample cup, and the amount of sample is such that the probe is completely immersed. The diameter of the sample cup must ensure that the probe has enough space. In the mechanical method, a small volume sample can be tested using a platinum rod method. Platinum rods are small in diameter and can be used with small radius sample cups, so the amount of sample required will be less than for large sample cups. However, the measurement accuracy of platinum rods is lower than that of plates and rings. Therefore, for small volume samples, only 10 μL of sample is required for the hanging drop method, which is most suitable for small volume sample measurement.
Whether it is a surfactant sample <br> Testing surfactant samples will have interesting findings. As shown in Table 1, there are two standards for measuring the surface tension of a surfactant sample. These two standards are tested using the plate method and the ring method, respectively. However, the cyclic method for testing surfactant samples has some problems because of the pulling of the surface during the measurement. This means that it is difficult to reach equilibrium during the test due to the continued creation of new surface areas during the pull. The test results will be larger than the results in equilibrium.
On the other hand, the plate method is easy to obtain a stable state because the plate is stable in the test liquid during the test. However, the surface of the plate will cause adsorption and change the wettability of the plate. For the above reasons, the hanging drop method is the most suitable method for measuring the surface tension of a surfactant sample unless certain criteria must be followed. Another reason to choose the mechanical test is to be able to automatically measure the critical micelle concentration.
Viscosity of the test liquid <br> Since the mechanical test is based on the wettability between the liquid and the probe, for example, a meniscus can be formed, the viscosity of the liquid needs to be taken into consideration. When the viscosity of the liquid is too high, it takes longer to wet the probe. This is easy to implement for the board test because the board method will have a long time to balance during the test. Since the viscous sample is difficult to control the tension of the ring, the ring method is not suitable for testing liquid samples with high viscosity. Although the hanging drop method can measure a sample having a viscosity greater than 1000 mPa*s, since the sample density also has an influence on the test, the limit of the viscosity sample cannot be determined. When considering whether the test method is suitable for the sample surface tension test, it is necessary to test in advance the flow state of the liquid when the sample is poured into the sample cup. The viscosity values ​​of some common liquids can be seen in Table 2.
Whether sample contamination is easy to clean When using mechanical methods to determine surface and interfacial tension, cleaning of sample cups and probes is critical. The probe is initially rinsed with a suitable solution and then baked with an alcohol lamp to ensure good wettability of the probe during the test. The sample cup can be cleaned with the same solution and rinsed with deionized water for next use. In many cases, the cleaning time of the probe and sample cup is much longer than the test time. In order to avoid the time consumption of the cleaning process, optical measurement can be used, and the pipette tip can be discarded after use.
Whether temperature control is required during measurement <br> For many materials, the surface tension is affected by temperature. If this is the case, temperature control is a must. For example, if the nature of the surfactant during the laundry process is studied, then temperature control is required during surface tension measurement.
On the other hand, for biological samples, the temperature is controlled at 37 ° C in order to obtain accurate results. Usually the value of the surface tension should indicate the test temperature. Both the optical method of the Theta device and the mechanical Sigma device are capable of temperature control, but Sigma temperature control is more convenient.
Whether to control pressure during the measurement process <br> In some applications, especially in the original mining process, pressure plays an important role. Since surface and interfacial tension are pressure dependent, it is necessary to test for such samples under specific pressure conditions. Theta high pressure accessories are available for temperature and pressure control.
Whether to measure other parameters <br> Both the optical method of Theta and the mechanical method of Sigma can measure other indicators in addition to the test surface and interface tension. If both methods are suitable for your sample test, you may want to consider whether other measurements are useful to you. If the surfactant sample is frequently measured, a critical micelle concentration test may be required, and the critical micelle concentration concentration can be tested in a mechanical Sigma device. Contact angles can also be tested with tension gauges. However, the optical method is the optimum method for measuring the contact angle. The Theta device of the optical method can also be equipped with an oscillating drop module to meet the interface rheology measurement. This parameter is very important for foaming materials and latex materials.
Summary <br> The best measurement method for your application depends on the sample characteristics and target test conditions. It is also necessary to consider whether other parameter measurements are needed and whether there will be new application requirements in the future. As shown in Table 3, the optimum measurement methods for different samples and different equipment are listed.
In this application article, we will provide a tool for method selection. By asking questions about the sample and application direction, you can determine which method is best for the test.
Quickly view measurement methods
The most commonly used methods for measuring surface tension and interfacial tension are the ring method, the plate method, and the hanging drop method. The first two are mechanical methods, and the last one is optical method.
The mechanical method is to measure the change in force between a gas and a liquid or between a liquid and a liquid at a probe position. The probe is connected to a high precision balance and the liquid interface is in contact with the probe. The balance is calculated by the balance of the force when the probe is in contact with the liquid interface to calculate the surface tension. The force depends on factors such as the shape and size of the probe, the contact angle between the probe and the liquid, and the surface tension of the liquid. The shape and size of the probe are easy to control. The probe is usually made of platinum to ensure that the probe's contact angle with the sample is close to 0 degrees. Both probes are often used in tests: plates and rings. Platinum rods can also replace plates when the amount of sample is limited.
Surface tension and interfacial tension can also be measured using the hanging drop method in the optical method. The shape of the liquid suspended at the needle depends on the balance of forces, including the surface tension of the test liquid. Modern computing uses an iterative method to compute the solution of the Young-laplace equation. Therefore, the surface and interfacial tension of two incompatible liquid bodies of known density can be calculated.
Method selection problem
There are a few things that need to be considered in advance to choose the most appropriate measurement method for your application. Below we summarize some of the issues.
Is there a need for standardized measurement methods? The mechanical method is the most common test method for measuring surface tension in international standards, although some optical methods still exist. The list of tensile measurement standards is shown in Table 1. If your application needs to comply with industry standards, then you need to choose a test method based on the criteria.
Whether the sample volume is limited In some applications, especially biological samples, the sample volume is very limited due to the expensive or difficult to obtain samples. In this case, a small sample size measurement method must be selected.
In the mechanical method, the required sample amount is in the milliliter range. The exact amount of sample is determined by selecting the sample cup, and the amount of sample is such that the probe is completely immersed. The diameter of the sample cup must ensure that the probe has enough space. In the mechanical method, a small volume sample can be tested using a platinum rod method. Platinum rods are small in diameter and can be used with small radius sample cups, so the amount of sample required will be less than for large sample cups. However, the measurement accuracy of platinum rods is lower than that of plates and rings. Therefore, for small volume samples, only 10 μL of sample is required for the hanging drop method, which is most suitable for small volume sample measurement.
Whether it is a surfactant sample <br> Testing surfactant samples will have interesting findings. As shown in Table 1, there are two standards for measuring the surface tension of a surfactant sample. These two standards are tested using the plate method and the ring method, respectively. However, the cyclic method for testing surfactant samples has some problems because of the pulling of the surface during the measurement. This means that it is difficult to reach equilibrium during the test due to the continued creation of new surface areas during the pull. The test results will be larger than the results in equilibrium.
On the other hand, the plate method is easy to obtain a stable state because the plate is stable in the test liquid during the test. However, the surface of the plate will cause adsorption and change the wettability of the plate. For the above reasons, the hanging drop method is the most suitable method for measuring the surface tension of a surfactant sample unless certain criteria must be followed. Another reason to choose the mechanical test is to be able to automatically measure the critical micelle concentration.
Viscosity of the test liquid <br> Since the mechanical test is based on the wettability between the liquid and the probe, for example, a meniscus can be formed, the viscosity of the liquid needs to be taken into consideration. When the viscosity of the liquid is too high, it takes longer to wet the probe. This is easy to implement for the board test because the board method will have a long time to balance during the test. Since the viscous sample is difficult to control the tension of the ring, the ring method is not suitable for testing liquid samples with high viscosity. Although the hanging drop method can measure a sample having a viscosity greater than 1000 mPa*s, since the sample density also has an influence on the test, the limit of the viscosity sample cannot be determined. When considering whether the test method is suitable for the sample surface tension test, it is necessary to test in advance the flow state of the liquid when the sample is poured into the sample cup. The viscosity values ​​of some common liquids can be seen in Table 2.
Whether sample contamination is easy to clean When using mechanical methods to determine surface and interfacial tension, cleaning of sample cups and probes is critical. The probe is initially rinsed with a suitable solution and then baked with an alcohol lamp to ensure good wettability of the probe during the test. The sample cup can be cleaned with the same solution and rinsed with deionized water for next use. In many cases, the cleaning time of the probe and sample cup is much longer than the test time. In order to avoid the time consumption of the cleaning process, optical measurement can be used, and the pipette tip can be discarded after use.
Whether temperature control is required during measurement <br> For many materials, the surface tension is affected by temperature. If this is the case, temperature control is a must. For example, if the nature of the surfactant during the laundry process is studied, then temperature control is required during surface tension measurement.
On the other hand, for biological samples, the temperature is controlled at 37 ° C in order to obtain accurate results. Usually the value of the surface tension should indicate the test temperature. Both the optical method of the Theta device and the mechanical Sigma device are capable of temperature control, but Sigma temperature control is more convenient.
Whether to control pressure during the measurement process <br> In some applications, especially in the original mining process, pressure plays an important role. Since surface and interfacial tension are pressure dependent, it is necessary to test for such samples under specific pressure conditions. Theta high pressure accessories are available for temperature and pressure control.
Whether to measure other parameters <br> Both the optical method of Theta and the mechanical method of Sigma can measure other indicators in addition to the test surface and interface tension. If both methods are suitable for your sample test, you may want to consider whether other measurements are useful to you. If the surfactant sample is frequently measured, a critical micelle concentration test may be required, and the critical micelle concentration concentration can be tested in a mechanical Sigma device. Contact angles can also be tested with tension gauges. However, the optical method is the optimum method for measuring the contact angle. The Theta device of the optical method can also be equipped with an oscillating drop module to meet the interface rheology measurement. This parameter is very important for foaming materials and latex materials.
Summary <br> The best measurement method for your application depends on the sample characteristics and target test conditions. It is also necessary to consider whether other parameter measurements are needed and whether there will be new application requirements in the future. As shown in Table 3, the optimum measurement methods for different samples and different equipment are listed.
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