# Goniometer

Digidrop GBX (Contact Angle Meter)
Surface tension and contact angle measurement between the liquid and the surface are used to predict the wettability, to calculate the spreading coefficient and the surface energy.
Surface Tension
To know the surface tension between the liquid and the surface,
• Interfacial tension between the vapor and the surface is measured using the reference liquids (Water, glycerol, diiodomethance) put on the surface to be analyzed
• Interfacial tension between the liquid and the vapour is measured by pendant drop method
• Contact angle is measured by putting the liquid on the surface.
Hence, surface tension between the liquid and the surface can be calculated.
Contact angle
Contact angle is defined as the angle formed by the liquid-solid and liquid-liquid interfaces when liquid is made to contact the solid surface. This is an important parameter which reveals the surface tension and surface energy values. If the contact angle is greater than 90 Deg, the liquid forms droplets on the solid surface and if it is less than 90 Deg, the liquid spreads out. Contact angle tends to zero when the liquid forms thin film on the surface.
Working principle
Goniometer explained below is of optical type that is geometry of the drop is captured and analyzed.Drop shape analysis is the convenient way to measure contact angle and surface tension.
In this, fluid to be analyzed is loaded using a syringe and image is grabbed either as a single snap shot or as a series of shots leading to a movie. The movie is stored and treated as a database. Data is digitized by the frame grabber as an 8-bit gray scale. Gray scale images are analyzed for linear dimensions, contact angle and surface tension, plus other data that can be derived from these results.
Surface tension is measured by fitting the shape of the drop to the Young-Laplace equation. Contact angles are measured by fitting a mathematical expression to the shape of the drop and then calculating the slope of the tangent to the drop at the Liquid-Solid Vapor interface line.
Principle assumptions
• The drop is symmetric about a central vertical axis this means that it is irrelevant from which direction the drop is viewed.
• The drop is not in motion in the sense that viscosity or inertia are playing a role in determining its shape: this means that interfacial tension and gravity are the only forces shaping the drop.