The components of a whole contact angle measurement are:
- Precise optical-mechanical structure.
- Optical imaging system
- Precise titration system
- Professional Interface Chemical Analysis Software.
The instrument can be employed for the testing dynamic and contact angle values, distribution of solid surface free energy, liquid-gas and liquid-liquid interfacial tension value, and liquid interface viscoelastic index. Moreover, it allows the measurement of contact angles of various materials and powder sample determination post tableting. Surface tension, droplet shape size, advancing angle, etc., can also be evaluated through whole contact angle measurement.
What is the contact angle?
The angle is between the tangent line of the gas-liquid interface, liquid and solid three phases on the liquid side, and the solid-boundary line. The value is an indicator of the wettability of the liquid. If the value of the contact angle is low (value of 0) °, the wettability is considered on the good side, and the liquid can spread easily on a solid surface. However, the wettability of a liquid is poor if the contact angle is high (value of 180°); it cannot spread evenly across the surface. Intermediate wettability allows the liquid to be distributed but not completely.
Measurement methods for Contact Angle:
There are two methods available for the determination of liquid and solid surface contact angles:
- Balance weighing method.
- Optical method.
A balance weighing method is an indirect approach to measuring the contact angle. It’s convenient only for solid surfaces with regular geometric shapes. The optical method is a direct approach based on the observation of the contact angle of a droplet on a solid surface. The determination can be done quickly regardless of the geometrical shape or size. It is highly applicable with multiple measurement modes that can be carried out under the same set of conditions. The discovery of cameras and digital image processing has improved the optical video method by optimizing automation precision, accuracy, and measurement speed.
What’s the difference between a contact angle measuring apparatus and the traditional contact angle method?
The traditional contact angle measurement is based on manually quantifying diameter area and other similar droplet parameters after diffusion. Instability of the measurement time point, accuracy, and irregularity of the shape make calculation manually an inadequate approach.
The instrument employed pours the fluid according to the amount of equipment to achieve stability and consistency of volume and speed. It records droplet images at various intervals, ensuring stability and repeatability of the detection environment and image acquisition time points. Image analysis and fitting of the droplet contour edge allow scientific and quantitative determination of the droplet contact angle.
What is the difference between a contact angle and a drop angle meter?
The difference between a drop angle meter and a contact angle meter is:
- Probe Liquid:
The drop angle meter employs distilled water, whereas the contact angle meter utilizes more probe liquid. The use of probe liquid leads to different designs such as:
Distilled water in the liquid inlet device of the drop angle meter allows a disposable plastic liquid inlet tube, plastic needle, or a jet needle made of plastic materials to be used. Consequently, compatibility and cleanliness issues come under consideration. Professional applications such as wettability envelopes are typically overlooked while developing software, resulting in a limited application range.
- Different software algorithms
The water drop angle measuring instrument offers a better approach to evaluating the cleanliness of the probe liquid. The basic select plane algorithm can be employed without the need for the Asha algorithm. Since the contact angle measuring instruments uses a probe, the fluid changes more. As a result, the professional Asha algorithm must be required to match in the test algorithm. These instruments are widely used for the determination of the physical and chemical properties of solid materials.
Factors Affecting the Measurement Accuracy of the Contact Angle Meter
- Correctness Impact of Baseline Level Confirmation
The horizontal baseline is bound to influence the process of contact angle testing. The secret to determining the horizontal line in the contact angle tester testing process includes good image acquisition technology, an excellent optical system, and an experienced operator.
- Influence of surface cleanliness and uniformity
Cleanliness is an essential aspect of contact angle measurement. If the results differ by a significant number, then the error can be due to the surface rather than anything else.
- Sample impact
Multiple factors such as dynamic balance and temperature can affect the contact angle value and must be considered. However, it’s generally most affected by the shape of the sample. If the sample is a curved surface, it must be corrected. Ideally, large samples are not preferred for standard equipment of contact angle tester.
Factors to consider when using:
The two prominent factors that should be considered while using the instruments are:
- Equilibration time.
- Constant System Temperature.
The contact angle can change due to insufficient equilibration, which results in a dynamic contact angle phenomenon. Low viscosity fluids achieve equilibrium rather rapidly, and the usual experimental manipulation can obtain the equilibrium value.
The contact angle is affected by temperature. Therefore, the contact angle also changes when the interfacial tension changes with temperature. For a typical system, the value of dθ/dT is 0–0.20°/K around room temperature, so the influence of temperature is not concerning. Additionally, suppose the temperature changes by 1–2K; the effect on contact angle is insignificant. In that case, these considerations should be made by the unit employing the contact angle tester during the process of purchasing and applying the contact angle tester, or other changes will reflect in the final data. The variation can make it hard to analyze the data.
What methods measure dynamic contact angle with a contact angle meter?
The static contact angle measurement is not enough to evaluate surface wetting properties. Dynamic contact angle must be determined to obtain complete information regarding the wetting properties of the system under test. Contact angle measurement is more useful on actual material than on an ideal surface.
Three methods are available for contact angle measurement using a contact angle meter:
Inclined plate method: A droplet is placed on the sample’s surface and slowly tilted on an inclined table. Any changes in the shape, contact angle, and position of the droplet are observed. The characteristic property of the method is that it can measure the changes in the advancing and receding angle along with the rolling angle of the droplet on the surface.
Centrifugal turntable method: A droplet is placed on the sample’s surface and rotated using the centrifugal turntable. Any changes in the shape, contact angle, and position of the droplet are observed. The characteristic property of the method is that it measures the changes in the advancing and receding angle and the holding force of the droplet on the surface. It applies to hydrophobic ad hydrophilic materials.
Liquid addition and subtraction method: A droplet is placed on the sample’s surface, followed by an injection needle’s insertion into the droplet. The fluid is slowly transferred to the droplet, increasing the volume by a certain degree. The liquid is later slowly sucked back to reduce the volume. Any changes in the shape, contact angle and position of the droplet are observed at the same time. The characteristic property of the method is that it measures the changes in the advancing and receding angle without the need for unique accessories. However, it affects the droplet shape, and the injection needle, leading to errors in contact angle calculations.
How to choose a contact angle meter?
Various contact angle measuring instruments are available per the users’ requirements. It might be overwhelming to decide on one type. So, which contact angle meter to choose?
- Closely look at the details to see what it measures; performance of solid with the same liquid or performance of liquids with the same solid. If the liquid is relatively constant, it’s easier to operate the peristaltic pump knob to add the sample. Suppose the fluid is regularly changed, viscous, difficult to clean, or corrosive. It is recommended to use a conventional micro-injector or disposable sampler and supporting fixtures, which are also knob-type injections.
- Evaluate the user’s demand for a surface tension test in the attached function. The pendant drop method, attached to some contact angle measurement instruments, is a classic and accurate surface tension test method with high operator requirements. If multiple testers need to evaluate surface interfacial tension in batches, an automatic tensiometer is recommended.
- Certain instruments offer three screenshot methods:
- Instant Freezing
- Automatic continuous photography
- High-speed photography.
These methods offer measurement accuracy, even for volatile liquids.
- No direct relationship exists between contact angle and droplet size. 3–10 liters sample volume is ideal. Tiny droplets will affect the volatilization factor, whereas large droplets will affect the gravitational factor. The use of fully automatic precise injection can avoid specific problems.
- When powder testing for contact angle infrared tablet press should be used to allow stress-free demolding after the sample is pressed. A powder solid contact angle measuring instrument should be utilized if the sample cannot be tableted.