Tension Superficial

CAPILLARY RISE

Learning Objectives

Study Guide

What is capillarity?

 

Capillarity is the tendency of a liquid to be absorbed into a tube of small diameter.  This phenomenon is responsible for the tendency of porous media to absorb water, such as a sponge, fabric or soils.  Note that pores must be communicated for water to flow through the media.

Observe the phenomenon:    

Capillary rise in soils of different texture

How does capillarity occur with water?

When a glass capillary tube is placed in contact with a water surface, the static contact angle between glass and water is established. Surface tension around the perimeter of the tube results in a force with a vertical component that drives water upwards.  The movement continues until the force due to surface tension equals the weight of the water column.

See an illustration:    

Free body diagram of water in a capillary tube

The concave meniscus formed at the surface of the water column is a result of the adhesive forces being greater than the cohesive forces. When cohesive forces are greater than adhesive forces the meniscus is concave, as is the case with glass and water.

See an illustration:    

Meniscus for water and mercury in glass

Role in irrigation

• Surface tension and pore size and distribution play a key role in soil water retention.

Soil moisture retention curves in soils

 

• Capillarity plays an important role in irrigation, especially in systems that rely on this property to distribute water in the root zone.  For example in subirrigation, furrows and especially in drip irrigation.

Video of infiltration from a drip source

 

• To estimate wetted bulb dimensions in a deep homogeneous soil use the software below.

Software for estimating wetted bulb dimensions from a drip source (Sepúlveda y Zazueta, 2004)

RESOURCES