When a defined droplet is applied to a solid medium (here: skin), the liquid forms a certain shape (drop shape). The point at which the skin, liquid and air meet determines the contact angle.
The relationship between the contact angle, the skin surface free energy, the liquid surface tension and the interfacial tension between skin and liquid is defined by Young’s equation:
γS = γL cosθ + γSL
θ: Contact angle
γS: Surface free energy of the solid
γL: Surface tension of the liquid
γSL: Interfacial tension between solid and liquid
The larger the contact angle the lower the wettability, the lower the contact angle the better the wettability.In literature the relation between wettability of a substance and contact angle are defined as follows:
- Contact angle < 90° = hydrophilic surface
- Contact angle > 90° = hydrophobic surface
Translated for the skin,
higher wettability indicates more moisture at the skin surface. It is also linked to the sebum content of the skin as
sebum makes the skin more hydrophilic. Dry skin shows correspondingly lower wettability. Skin surface free energy and wettability are also related.
Skin with lower surface energy generally shows lower wettability properties.
high | contact angle | low |
low | wettability | high |
low | adhesiveness | high |
small | skin’s free energy | large |
With the Drop Angle Meter, the inner forearm is placed on a comfortable rest, which can be moved vertically and tilted in order to have the skin surface where the drop is applied to absolutely levelled to prevent the water from running or disintegrating. The compact system features a very uniform diffuse LED light source and a high-resolution camera, which takes the macro image of the drop on the skin from the side, which is then analyzed in the software. All data, images, and even videos are saved in the database and can conveniently be filtered and exported.
The camera can be moved in x, y and z direction (up and down as well and back and forth and vertically), to perfectly match the best depth of focus for each drop size. The solid system features height adjustable feet to compensate unevenness of the working surface.