BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Surface Tension
A molecule in the center of a liquid is completely surrounded by other molecules and is attracted
equally in all directions. A molecule at the air/liquid surface is attracted inwards, as a result of the
lower concentration of attracting molecules in the air relative to the liquid. This will set up an
imbalance of force on a surface molecule and energy must be expended in order to move molecules
into the surface from the bulk liquid. The amount of energy required to move enough molecules to the
surface to create a unit area of surface is called the surface energy.
As a consequence of this imbalance of force, the surface of a liquid always tries to contract to the
smallest possible area. As a result of this tendency to contract, the surface behaves as if it were under
tension (see Figure 5-6). If an imaginary cut were made along the surface, a force would have to be
applied to hold the two portions of the surface together. The force would be proportional to the length
of the cut and would act at right angles (perpendicular) to the cut. The magnitude of the force per unit
length is called the surface tension and is numerically equivalent to the surface energy. Surface
energy/tension, acting to minimize surface area, causes soap bubbles and water droplets to form
spheres (minimum surface area). Surface energy/tension also accounts for the capillary rise of water
in small glass tubes.
Figure 5-6
Diagram of Attraction Forces in Liquids
Polar molecules, such as water, because of strong dipolar attractive forces between the molecules,
show large imbalances of force at the air/liquid surface. This strong imbalance of surface forces
results in a high-surface energy/tension for water. Ionic solids, with even stronger ionic and
coulombic attractive forces, show even larger imbalances of force at the air/solid surface.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 5-11