Contact angle measurements are used to characterize wettability. It is utilized in a wide range of applications from biomedical coatings to enhanced oil recovery and printed circuit boards. In most applications, the measurement of a sessile drop on a material surface is used to determine the wetting state of the material. Typically, the sessile drop of a few microliters in size is used. Depending on the contact angle formed between the liquid and the surface, this leads to a drop baseline width from around a millimeter to few millimeters. For most of the applications, this size range is suitable, and several droplets can be made on the sample. However, in some cases, the contact angle measurement needs to be done in an area way smaller than one millimeter in diameter. For these types of applications, the picoliter dispenser should be used.
Picoliter dispenser is based on piezo-driven inkjet printing technology where an integrated piezo-actuator induces a shock-wave which causes a droplet to be emitted from the nozzle. The size of the droplet is dependent on the inner nozzle diameter as well as the fluid properties (such as density, viscosity, and surface tension of the fluid). As small as 20 pl drops can be produced which leads to a drop diameter of less than 100 um (depending on the contact angle). Even larger drops of around a few 100 pl will typically lead to well below 0,5 mm baseline width.
When selecting a suitable nozzle size for your liquid, in addition to the desired drop size, the viscosity of the liquid needs to be considered. As the inner nozzle diameter of the picoliter dispenser can be as low as 30 um, the high viscosity liquids will have difficulties passing through such a small orifice. When sample viscosities are high, the nozzle size needs to be increased to 50 um, or even 70 um, but in addition heating of the liquid needs to be added to increase its flowability.
As with any standard dispenser, the picoliter dispenser can be used together with the automated XYZ -stage. This allows you to do several contact angle measurements in a row by using picoliter drops with one click of a button. In the video below, the picoliter dispenser is used to make 3 drops in a row at 0.5 mm apart from each other.
Hydrophobic coatings are used in many consumers products. Contact angle measurements can be used to study them.
HMDS is used to render the surface of silicon more hydrophobic and thus to match better with the chemistry of the photoresist.
Quality control is needed to detect any possible problems early on and contract angle measurement provides an excellent tool for that.