Contact angle measuring instrumentIt is an instrument used to measure the contact angle between liquids and solids, mainly used to evaluate the wettability of liquids on solids, and then analyze the physical and chemical properties of solid surfaces such as wettability, cleanliness, uniformity, and surface free energy. It has a wide range of applications in materials science, chemistry, biology, microelectronics, coating technology, and other fields.

This article will comprehensively analyze the contact angle measuring instrument from the following aspects:

1、 Basic concept of contact angle

2、 Working principle of contact angle measuring instrument

3、 The main types of contact angle measuring instruments

4、 Composition and key components of contact angle measuring instrument

5、 Measurement method of contact angle

6、 Steps for using a contact angle measuring instrument

7、 Data Processing and Analysis

8、 Precautions and application areas

1、 Basic concept of contact angle

definition:

Contact Angle refers to the angle between the tangent of the droplet contour and the solid surface at the junction of liquid-solid, liquid-solid, and solid gas phases when the droplet reaches equilibrium on the solid surface. It is usually represented by θ.

Judgment criteria:

θ<90 °: Hydrophilic (liquid wets solid surfaces, such as water droplets on clean glass)

θ> 90 °: Liquid repellent (liquid does not easily wet solid surfaces, such as mercury on glass)

θ ≈ 0 °: Wetting

θ ≈ 180 °: non wetting (superhydrophobic)

Application significance:

By measuring the contact angle, the wettability, roughness, chemical composition, and surface treatment effect of the material surface can be inferred.

IIWorking principle of contact angle measuring instrument

The contact angle measuring instrument calculates the shape parameters of droplets based on optical imaging principles and geometric/mechanical models, thereby inferring the size of the contact angle. The basic workflow is as follows:

Add trace droplets (usually 1-10 μ L) onto the surface of the solid sample to be tested;

Using high-resolution cameras to capture droplet images;

Software performs edge detection on images and extracts droplet contours;

According to the droplet contour curve, use fitting algorithms (such as circle fitting, ellipse fitting, Young Laplace equation fitting, etc.) to calculate the contact angle;

Output measurement results and further analyze parameters such as surface energy.

3、 The main types of contact angle measuring instruments

Static contact angle measuring instrument: measures the contact angle of a liquid droplet in a stationary state.

Dynamic contact angle measuring instrument: By tilting the sample stage or increasing/decreasing the droplet volume, measure the forward and backward angles to study wetting and hysteresis phenomena.

Suspended droplet method surface energy analyzer: used to calculate the surface energy of solids by measuring the shape of suspended droplets and inferring the surface tension of liquids.

Interface rheometer type contact angle meter: Combining dynamic contact angle and interface expansion rheological properties, it is suitable for studying the interaction between viscoelastic liquids and interfaces.

4、 Composition and key components of contact angle measuring instrument

Sample stand: used to fix the solid sample being tested, usually with horizontal adjustment and heating functions.

Liquid addition system: Micro injection pump or manual injector, precise control of droplet volume.

Image acquisition system: CCD or CMOS high-definition camera, combined with microscope lens to obtain droplet images.

Light source: Provides uniform and stable backlight (commonly LED cold light source), enhancing the contrast of droplet edges.

Sample environment control system (optional): temperature control chamber, humidity control, atmosphere control, etc.

Analysis software: used for image processing, contour extraction, contact angle calculation, data export, and report generation.

　　
5、 Measurement method of contact angle

Sessile Drop Method - commonly used

Drop the liquid droplet onto a solid surface and measure the static contact angle after it stabilizes.

Single droplet and multiple droplet (different liquids) tests can be conducted to compare surface wettability.

Tilting Method

After the droplet stabilizes, slowly tilt the sample stage and record the changes in contact angle between the leading and trailing edges of the droplet to obtain the advancing angle and retreating angle.

Used to evaluate contact angle hysteresis and surface non-uniformity.

Needle in/out Method

Insert or withdraw the liquid using an injection needle, change the droplet volume, and observe the angle change when the contact line moves.

Also used to measure the forward and backward angles.

Pendant Drop Method

Used to measure the surface tension of liquids and indirectly calculate the surface energy of solids.

Not directly measuring contact angle, but commonly used in surface energy analysis.

Wilhelmy Plate method (hanging plate method)

Calculate the contact angle and surface energy by measuring the force changes when a solid sheet is immersed in a liquid.

Suitable for flexible materials such as fibers and films.

6、 Steps for using a contact angle measuring instrument

Power on preheating: Turn on the light source, camera system, and computer, and preheat for 10-30 minutes to ensure stability.

Sample preparation:

Ensure that the surface of the sample is clean, flat, and free from contamination.

Can be cleaned with solvents (such as ethanol, acetone) and blown dry.

If high temperature testing is required, install the sample to the temperature control table.

Calibration instrument:

Adjust the level of the sample stage (using a spirit level or software built-in calibration tool).

Focus on the image to ensure clear edges of the droplet.

Droplet addition:

Use a microsyringe to add a set volume of liquid droplets (recommended 1-5 μ L).

Avoid touching the surface of the sample with the needle.

Image acquisition:

Wait for the droplet to stabilize (usually 5-30 seconds) and take a clear image.

Multiple frames can be captured continuously to improve accuracy.

Angle calculation:

The software automatically recognizes the contour of the droplet and selects a fitting model (such as circle, ellipse, Young Laplace).

Read the left and right contact angle values, take the average or record separately.

Dynamic testing (optional):

Tilt the sample stage or change the droplet volume, and repeat the above steps to measure the forward/backward angle.

Data storage and analysis:

Export data such as images, angle values, and fitting curves.

Perform statistical analysis or surface energy calculation.

Shutdown cleaning: Turn off the device, clean the sample stage and injection needle, and record usage.

7、 Data Processing and Analysis

Contact angle calculation model:

Circle fitting: suitable for small contact angles (θ<30 °) and droplets approaching a spherical shape.

Elliptical fitting: suitable for medium contact angles (30 °<θ<120 °), considering the influence of gravity.

Young Laplace fitting: Accurate, suitable for large droplets, large contact angles, or complex shapes, considering surface tension and gravity balance.

Surface energy estimation:

Using Owens Wendt, Van Oss, Zisman and other methods, the solid surface energy and its polarity/dispersion components were inferred from various liquid contact angle data.

Contact angle hysteresis analysis:

Lag=forward angle - backward angle, reflecting the influence of surface roughness, chemical inhomogeneity, or adsorption layer.

8、 Precautions and application areas

Notes:

The surface of the sample must be clean and dry to avoid contamination and errors.

The volume of the droplet should be moderate, with significant influence from gravity if it is too large, and blurred edges if it is too small.

Environmental vibrations, airflow, and temperature fluctuations can affect measurement accuracy and require operation in a stable environment.

Take the average of multiple measurements to improve repeatability.

The results of different fitting methods may vary greatly, and an appropriate model should be selected based on the droplet shape and theory.

Application fields:

Evaluation of material surface modification (such as plasma treatment, coatings, nanostructures)

Research and development of superhydrophobic/superhydrophilic materials (such as self-cleaning surfaces, anti fog glass)

Compatibility analysis of biomaterials (such as cell adhesion, protein adsorption)

Oil water separation and microfluidic device design

Optimization of printing, coating, and bonding processes

Quality inspection of semiconductor wafer cleaning

Research on the Spread of Cosmetics and Pharmaceutical Preparations

summary

The contact angle measuring instrument is a high-precision, non-destructive surface analysis tool, whose core is to capture the droplet morphology through optical means and calculate the contact angle by combining mathematical models. Mastering its working principle, correct usage methods, and data analysis skills is crucial for evaluating and optimizing the surface properties of materials. With the advancement of technology, modern contact angle measuring instruments have integrated functions such as automation, high temperature and high pressure, and environmental control, becoming equipment in scientific research and industrial quality inspection.