Abstract: Contact angle is a basic physical variable of the three-phase interface. Unsaturated soil is a typical three-phase matter. The research on contact angle in unsaturated soil is at the beginning stage. In general, contact angle is supposed as a constant or overlooked. Actually the contact angle will change with the external environment. The contact angle of sand will increase with surface modification. The surface property of sand is altered from hydrophilic to hydrophobic. The apparent contact angle of sand increases by mixing the hydrophobic sand into normal sand. The mixture is used for preparation of samples. The suction measurement tests on samples with different contact angles are conducted. The results indicate that the contact angle has important effect on matric suction in unsaturated soil. When the water content at the same value, the matric suction decreases with the increasing of contact angle. When the water content reaches a certain value, matric suction would not be affected by the contact angle of sand (under hydrophilic condition). When the contact angle reaches a threshold value, the matric suction disappears. The negative pore water pressure does not exist in unsaturated soil.

Key words: contact angle, hydrophilic sand, hydrophobic sand, matric suction