Cohesive forces between liquid molecules develop surface tension. The molecules at surface are not surrounded by other similar molecules from all sides and cohere more strongly to those directly associated with them on the surface. This forms a surface "film" that restricts free movements of any object through the surface as compared to bulk phase. Surface tension is measured in dynes/cm, the force in dynes required to break a film of length 1 cm. it can be a surface energy in ergs per square centimeter, water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol.
Cohesion: Intermolecular attractive forces between similar molecules are cohesive forces. A water droplet is held together by cohesive forces and strong cohesive forces at the surface constitute surface tension.
Adhesion: Intermolecular forces between dissimilar molecules are adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces and then cause an upward turning meniscus at the walls and contribute to capillary action. The attractive forces between molecules in a liquid denoted as residual electrostatic forces and also called van der Waals forces.
Cohesion and Surface Tension
The cohesive forces between molecules liquid phase are shared with all neighboring atoms and on the surface have no neighboring atoms above. It exhibit stronger attractive forces upon their nearest neighbors on the surface and the intermolecular attractive forces strengthen surface tension.
Surface Tension of Water
The surface tension water is 72 dynes/cm at 25°C that takes a force of 72 dynes to break a surface film of water 1 cm long. The surface tension arises from the polar nature of the molecules. The surface tension of water decreases with temperature and hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to enter pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.
Viscosity is a measure of a resistance of a fluid which is being deformed by shear stress, in general the viscosity is "thickness". Thus, water is "thin", with lower viscosity while vegetable oil is "thick" with a higher viscosity. Viscosity describes a fluid's internal resistance to flow and is as a measure of fluid friction. Real fluids cause some resistance to stress and a fluid with no resistance to shear stress is an "ideal fluid" or "inviscid fluid". The study of viscosity is known as rheology.
Dynamic viscosity (absolute viscosity) determines the dynamics of an incompressible Newtonian fluid nad Kinematic viscosity is the dynamic viscosity divided by the density for a Newtonian fluid. Dynamic viscosity is referred to as viscosity, at room temperature, water has viscosity of 1.0×10-3 Pa∙s and motor oil 250 × 10-3 Pa∙s. in flow, layers move at different velocities and the fluids viscosity arises from the shear stress between the layers that ultimately oppose any applied force. Isaac Newton postulated that, for straight, parallel and uniform flow, the shear stress, τ, between layers is proportional to the velocity gradient, ∂u/∂y, in the direction perpendicular to the layers.
Dynamic viscosity: The η is viscosity in Pascal-second (Pa·s) is equal to kg·m−1·s−1. The CGS for dynamic viscosity is the Poise (P) named after Jean Louis Marie Poiseuille. It is denoted as centipoise (cP). Water at 20°C has a viscosity of 1.0020 cP. 1 P = 1 g·cm−1·s−1. The 10 P = 1 kg·m−1·s−1 = 1 Pa·s, 1 cP = 0.001 Pa·s = 1 mPa·s.
The CGS for kinematic viscosity is stokes (St), named after George Gabriel Stokes, and is also denoted a centistokes (cSt or ctsk). In U.S. 1 stokes = 100 centistokes = 1 cm2·s−1 = 0.0001 m2·s−1. 1 centistokes = 1 mm2·s-1 = 10-6m2·s−1.
Viscosity is independent of pressure and fall as temperature increases water viscosity goes from 1.79 cP to 0.28 cP from temperature range 0°C to 100°C. The viscosity of water is 8.90×10−4 Pa·s or 8.90×10−3dyn·s/cm2 or 0.890 cP at 25°C. The abovementioned fluid dynamics is studied together with survismeter