In this paper, the Q&A form has been used to describe the principle and the related technical points of interfacial tension mete. Interfacial tension measurement technology has been neglected by users for a long time. Therefore, we have this brief introduction, hope to help the users.

1.      Is the interface tension meter equal to the surface tension meter? What's the difference?

In general, an interfacial tension meter can be used to test surface tension and interfacial tension values. While, the surface tension meter is not necessarily could test the interface tension value. There are two concepts here: First, the surface tension usually means the interfacial tension between the liquid and the gas, and the interfacial tension usually refers to the liquid-liquid interface tension value, and sometimes also involves the liquid- Gas (high temperature and high pressure conditions) of the interface tension value. Thus, the surface tension value is usually higher than 16 mN / m, while the interfacial tension is sometimes as low as 0.0001 mN / m or even lower. Therefore, the requirements of the interface tension meter will be higher than the surface tension meter. Second, the measurement methods between the surface tension test and the interfacial tension test is very big. Surface tension testing usually use the platinum plate method and platinum ring method, while the interface tension test in addition to the above two methods, the most effective methods are the pendant drop method, the constraint sessile drop method and spinning drop method. In general, the surface tension meter and the interface tension meter are quite different.

On the other hand, the surface tension meter generally means the instrument which using the weighing principle, mainly using the platinum ring method and the platinum plate method. At present, the platinum ring method is usually sold as a subsidiary part of the platinum plate surface tension meter. The platinum plate surface tension meter is currently the market mainstream. The surface tension meter of the platinum plate method is divided into two categories, one is the classic algorithm based on zero buoyancy method and another one is based on the Young-Laplace equation correction method. The classical platinum plate method and the platinum ring method are both modified with zero contact angles as the prerequisite, but in fact, in the interface tension test, the contact angle of the range of changes is very large. It could be from 29 to 129 degree; therefore, zero contact angle assumptions in the two methods are invalid. Therefore, in addition to the Young-Laplace equation based on the surface tension meter, the other methods used to test the interface tension value will appear lower value. Specifically the interface tension between benzene and water could be used to verify (20 ℃, the interface tension is 35mN / m). Therefore, if you want to test the interface tension value, the preferred instrument is not a platinum plate method or platinum ring method based surface tension meter. At the same time, if the weighing principle based the surface tension meter is marked as an interfacial tension meter it might cause any misunderstanding.

2.      Which kind of interfacial tension meter is more suitable for testing the oil-water interface tension system with surfactants?

Due to the contact angle and the influence of buoyancy, the interfacial tension of oil -water, especially the systems containing surfactants, the measurement of dynamic interfacial tension value becomes very difficult. At present, the method of testing the interface tension includes pendant drop method, sessile drop method, spinning drop method, platinum plate method and platinum ring method. Combined with the application point of view, the platinum plate method test results slightly better than the platinum ring method, but the measured results are easy to be a little lower. If you only focus on the relative value of the change or repeatability test, you should consider the platinum plate method; the stability will be very good. However, if you want to get a better absolute value, pendant drop method and sessile drop method is better than spinning drop method and spinning drop method is better than platinum plate method. The reason is that the pendant drop method and the sessile drop method are more suitable for the saturate system operation, while the other methods cannot effectively achieve the saturate condition. Pendant drop method can measure the interfacial tension value down to 0.1mN/ m, and the constraint sessile drop method can reach 0.0001mN / m, which can be applied to a wide range of measurement. Both these two methods can test the value under the ultra-high temperature, high pressure, vacuum and other conditions.

In particular the pendant drop method can be used to test dynamic interfacial tension (surfactant adsorption at the interface). With high-speed cameras and pressure sensors, you can perform the high-precision Dead Time and bubble life tests when dripping water into the oil phase. At present, only the US Kino contact angle instrument SL200K and C60 series can provide the corresponding accessories.

Spinning drops interfacial tension meter can be used to test ultra-low interfacial tension values, but because of its low accuracy, it is usually used in the analysis of tertiary oil recovery. At present, the US Kino invented the ultra-high pressure, high temperature spinning droplet interface tension meter to solve the problem. It can simulate the underground operating environment. This technology of interfacial tension test is in a world leading level.

3.      What is the notice when choosing a spinning drop interfacial tension meter?

(1)    Temperature control technology for spinning drop interface tension meter: There are three main types of temperature control methods: control the surrounding air temperature of the sample tube, control the temperature of the heating source (current mainstream), and control the oil temperature which wrap the sample tube (Kruss, Germany). From the effect point of view, that control the surrounding oil method is the best way, but its drawback is that maintenance and cleaning is very inconvenient, and it’s easy to leak and difficult to inject the sample. The method that controls the heating source is not easy to control the sample temperature. There would be a big measurement error. Therefore, the air temperature control technology is the best way.

(2)    Temperature control range of the spinning drop interfacial tension meter: The temperature control range is determined by the test requirements of the instrument users. Such as most of Chinese oil fields’ demand is 80 degrees, but some areas of deep wells, the needs for temperature will be higher than 90 degrees. Currently, the air-heat type is the best heating method to control the temperature in spinning drop interfacial tension meter which provided by US-KINO. The heating method which controls the temperature of the heating device would make a big temperature gap between the sample tube and the target value. If the requirement of the temperature is higher than 100 degrees, a professional high temperature and high pressure spinning drop interface tension meter is needed.

(3)    Spinning drop interfacial tension meter motor control mode is mainly refers to the motor the brush DC motor and the brushless DC motor. DC brush motor has the advantage of low cost, but the shortcomings are obvious, the control accuracy is not high and the service life is relatively short. The brushless DC motor due to the abolition of the carbon brush, the service life will be very long, accurate reading accuracy could be up to 0.1RPM, while the price is relatively high.

(4)    The usage of the sample tube material and its seal structure: For the sample tube, currently, US KINO uses sapphire and quartz glass. Both of these materials maintain excellent performance in light transmittance, uniformity and refractive index. Some related instrument manufacturers provide the common silicate glass which has poor light transmission, poor uniformity and poor explosion-proof performance, the temperature and pressure resistance is also poor. Glass tube is almost-outdated technology. Sealing structure is divided into three categories: one side opening sample tube, two side opening self-sealed sample tube and dynamic sealed sample tube. From a technical point of view, the dynamic seal sample tube is the best, while the one side opening sample tube performs worst. However, from the practical application point of view, two sides opening sample tube is easier to be cleaned (ultrasonic cleaning and mechanical cleaning), it is easier to load crude oil samples and exhausted the air. Therefore the two sides opening tube is the preferred technology of spinning drop interface tension meter.

(5)    Software algorithm: spinning drop interface tension meter’s outline of the droplet is consistent with the Young-Laplace equation, which can be calculated by Young-Laplace equation technology. At present, the algorithm which invented by the US KINO has been authorized to Shanghai Solon in China. In fact, there is a more convenient test method called the simplified Fungernett method. This method needs to combine the sample tube thickness, that is, different surface or interface tension values of the sample using different diameter of the sample tubes, and as far as possible to form a droplet shape which length/width is larger than 1.5 times. At this point, Young-Laplace equation fitting technique is unnecessary. However, this method requires centrifugal force correction for the interfacial tension values of samples below 0.01 mN / m. The simplified Fungernett method based on centrifugal force correction is the most convenient and practical software algorithm for spinning drop interface tension meters. It is a remarkable fact that the Young-Laplace equation fitting method has higher accuracy.

4.      What is the main difference between the spinning drop interfacial meter, contact angle meter and interfacial tension meter with high temperature and high pressure?

High temperature and high pressure spinning drop interface tension meter can measure the ultra-low interfacial tension, while the high temperature and high pressure contact angle and interface tension meter cannot be used to measure the interfacial tension value which is less than 0.0001 mN/ m. This is the limitation of the pendant drop method and sessile drop method. In the specific application, each method has its own advantages. The high temperature and high pressure contact angle meter is especially suitable for testing the situation of the droplet property changes from hydrophilic to hydrophobic. Nowadays only US KINO can provide the multi-point test technology of droplet under high temperature and high pressure. The one point test technology can only test one droplet once which means new droplet would appear on the previous place that would be affected by the existed droplet. Therefore the core technology of the high temperature and pressure contact angle meter is to move the sample droplet. Another advantage of high temperature and high pressure contact angle meter and interface tension meter is that it can test the dynamic interfacial tension value under high temperature and high pressure conditions. Dropping the droplets at different speeds and using the high speed camera to take photos of the process of change, the dead time and the bubble life could be measured.

5.      Is it available to control the temperature when using interfacial tension meter to measure the oil/ water interfacial tension? 

Yes, the temperature can be controlled. The controlling technology includes constant temperature sink technology and semiconductor technology. At present, the temperature control range can reach -20 to 200 degrees Celsius; the accuracy is up to 0.1 degree. The further information you should contact US KINO or Shanghai Solon’s application engineers. The controlling range is also applicable to the spinning drop interface tension meter. Usually the commercial spinning drop interfacial tension meter only has the heating function; the cooling function must be combined with the rotation shaft technology and air-heat temperature control technology to have higher accuracy.

6.      Is there any accessory or special equipment needed when using pendant drop method to test the oil-water interfacial tension?

The accessories include: U-shaped needle, flat needle, PTFE needle and quartz glass square cylinder. If you need to control the temperature, the corresponding constant temperature sample pool or temperature control mechanism is also needed.

7.      What are the software algorithm used to measure the oil-water interfacial tension in pendant drop and sessile drop method?

At present, there are four software algorithms testing the interfacial tension values in pendant drop and sessile drop method. BA form method, the Young-Laplace equation fitting method which is based on the Select plane algorithm (the main instrument manufacturers are using this algorithm), ADSA-P method (A. W. Neumann) Young - Laplace Equation Fitting Technique of ADSA - RealDrop Algorithm (Shanghai Solon Invention Patent). Among them, Select plane algorithm is wildly used by many commercial instrument manufacturers, so its application is the most popular one. However, this algorithm is mainly used to test the contact angle under the conditions of super-hydrophobic. The most widely used in academic area is the ADSA-P algorithm. ADSA-RealDrop algorithm based on ADSA-P algorithm, using the second solution Young-Laplace equation algorithm, relative to the ADSA-P algorithm’s vertex curve radius empirical hypothesis, the ADSA-RealDrop’s scientific and reliability have improved.

8.      How to calibrate the optical contact angle meter and the spinning drop tension meter?

The ruby calibration ball is used to calibrate the contact angle. For spinning drop tension meter, ruby, ceramic or stainless steel scale all can calibrate the instrument, while the ruby calibration stone has the best accuracy. At the same time the thicker scale ruler is prefer, generally the proposed diameter is 1mm or 1.5mm.

9.      How to clean the sample tubes and the needles?

The ultrasonic wash machine is needed. After the ultrasonic cleaning, the secondary washing should use distilled water and sometimes ethyl alcohol is prefer. Finally using distilled water to measure a standard value to insure the tubes are clean.

10.  Is there any notification when using the pendant drop and sessile drop measurement?

(1)   Calibration is very important. The ruby calibration ball method is the most recommended.

(2)   The color camera is recommended as the edge of the image would have higher resolution. The accuracy would be high.

(3)   Cleaning is a must; especially the needle part’s cleaning after testing the sample with surfactant.

(4)   When using the pendant drop to test the surface tension the special flat tip needle is necessary, it should be made by PTFE or stainless steel material.