Basic knowledge of contact angle

Contact angle

The contact angle (θ) is the angle formed by a small drop of liquid when it meets the surface or interface of another phase (usually a solid)

Force balance at contact angle (θ)

Young's equation: In equilibrium, γ sv = γ sl + γ lv cosθ

In the formula, γ lv is the equilibrium between the liquid and its own vapor, γ sv is the equilibrium between the solid and its own vapor, and γ sl is the equilibrium between the solid-liquid interface.

static contact angle

The contact angle of a liquid droplet that is stationary on a solid surface and the three-phase boundary does not move is arbitrarily measured after dispensing, often referred to as the static contact angle.

dynamic contact angle

The time-dependent measurement of the static contact angle as the three-phase boundary moves is called the dynamic contact angle. There are two types of dynamic contact angles known as advancing contact angle and receding contact angle. The dynamic contact angle can be measured using two different methods; changing the volume of the droplet or using a tilting stand.

Advancing angle: The maximum contact angle θ a possible to obtain without increasing the area of the solid-liquid interface by dynamically adding liquid to the droplet.

Receding angle: If you start removing volume in the above case, then the smallest possible angle is called the receding angle θr

Lag Angle: The lag angle (θ H ) is the difference between the advancing angle (θ a ) and the receding angle (θ r ). θ H = θ a - θ r

Inclined plane method: The inclined plane method gives both advancing and receding contact angles in the same drop. Droplets are placed on the substrate and then gradually tilted. The advancing angle is measured at the front of the droplet, just before it starts moving. The receding contact angle was measured at the same time point on the back of the droplet.