If you have been looking for an instrument for contact angle measurement, you have probably noticed that measuring contact angle is not always enough. Although contact angle measurement does give you an indication on the wetting properties of the surface, it is always a value that depends on the measurement liquid used.
To better characterize the chemical properties of the solid surface, surface free energy measurements needs to be utilized. Whether you want to improve the adhesion of your coatings or study cell - biomaterial interactions, surface free energy can give valuable information about the chemistry of your surface.
Surface free energy measurement in practice
Although people often talk about surface free energy measurements, surface free energy or SFE for short, is not really measured but calculated. To calculate SFE, you need contact angle data of your solid surface. Most typically contact angle is measured with two different liquids. The selection of the measurement liquid is important for reliable results. Water is an obvious choice due to its non-toxicity and availability. Water also has high surface tension (72,8 mN/m) which makes it an ideal liquid for contact angle measurement as it forms an angle with most of the substrates (i.e. it does not wet the surface completely). Water is highly polar in nature thus it has a large polar component (51 mN/m) compared to many other liquids. The selection of the second liquid is much more cumbersome. It should be dispersive as using two polar liquids would exaggerate the polar component of the surface. The main problem with most of the dispersive liquids (like hydrocarbons) is that their surface tension is much lower than surface free energy of the solid surface making them wet the surface completely (i.e. contact angle is zero). Thus, more exotic liquids need to be used. Diiodomethane with the surface tension of 50.8 mN/m is the most used one.
Dual dispensing to speed up your measurements
As two liquids are required for SFE calculations, it is convenient if both liquids can be loaded prior to start of the measurements. This is possible with dual dispensing which utilizes two disposable tip dispensers. Disposable tips are filled with water and diiodomethane and the contact angle measurements can be done automatically. It is also possible to map a larger surface area easily. After measuring contact angles with both probe liquids, the contact angle data is send to SFE analysis that calculates the polar and dispersive components of your solid.
Watch this video on how dual dispensing can help your measurements: