Step 1: What is it......?
A contact angle instrument is a piece of equipment used to determine certain specific properties of liquids and solid materials as well as interactions between the two. These properties include “cohesive forces, adhesive behavior, wetting
properties, and morphological properties” . The “contact angle”, measured with this type of instrument, refers to the angle
created by the surface of a drop of liquid and a flat surface of a known material at the point of contact between the two. The angle is determined by the shape that the drop takes when placed onto the surface. This shape is produced by the interaction between the
properties of the liquid and the solid surface, which are determined by the relative surface tensions of the two materials . To be more specific, the cohesive behavior of the liquid serves to increase the contact angle by attempting to keep the liquid together in the drop shape, while the adhesion interaction between the two materials attempts to decrease the angle by trying to spreading the liquid across the surface of the solid material . It is important to note that the angle is always measured through the liquid , as seen in
The instrument is used to create the liquid drop, and placing it onto the solid, and then taking a picture of it. Then there is generally a piece of software used to help measure the actual contact angle from the image. So, any contact angle instrument consists of at least four components: a dispensing system, the stage, the viewing system, and the measurement system.
The dispensing system generally consists of a micro syringe to hold and dispense the liquid, and can be operated manually or by a motorized system.
The stage is that part of the instrument that hold the solid surface. It must be flat and level - usually built with the ability to adjust the tilt to a level position - so that the contact angle can be accurately measured (a tilted surface will change the observed contact angle; increased on the "down hill" side and decreased on the "up hill" side). The stage is generally able to move up and down a few inches to allow the liquid to be transferred from the syringe needle to the solid surface, and sometimes able to move laterally as well.
The viewing system generally consists of a microscope and/or camera to magnify and capture the image of the drop sitting on the solid, and a light source to illuminate the samples and enhance visibility of the outline of the drop.
The measurement system is the part that actually measures the contact angle from the image created with the viewing system, and is not a physical part of the instrument. This is usually done using a software program setup to trace the drop profile and calculate the angle at the contact point. With the correct calibrations these programs can provide additional information like drop volume, and contact area.