The forces acting on a liquid drop in contact with a surface can be represented as the surface energy of the solid, the surface tension of the liquid, and the energy remaining at the liquid-solid interface after the liquid molecules have interacted with the solid surfaces.
When a drop of liquid is in motionless contact with a surface, all of the forces acting upon the drop must be in equilibrium, and the forces acting in the plane of the surface must sum to zero.
Knowledge of surface energy is important in many industrial processes because of its direct influence on practical adhesion. Surface energies of solids may be measured using several techniques but the most common method is based on obtaining contact angle measurements of multiple fluids.
The Importance of Ballistic Deposition
Most contact angle measurements are made by simply depositing a liquid drop onto a surface from a syringe needle or small piece of tubing. These angles are generally in between an advancing and receding angle but are useful for comparative purposes.
The Surface Analyst™ doesn’t deposit a fully formed liquid drop, but rather constructs a drop on the surface by the coalescence of a pulsed stream of microdrops. This process is called Ballistic Deposition™ and facilitates wetting measurements on surfaces that are non-planar, textured, and non-horizontal.