adhesion

Meaning: Adhesion is the attraction between molecules of different substances, such as water and glass. Cohesion is the attraction between like molecules. Where a liquid meets a solid wall, whichever attraction is stronger sets the shape of the surface. If adhesion exceeds cohesion, the liquid wets the wall and forms a concave meniscus (water in glass). If cohesion exceeds adhesion, the meniscus is convex (mercury in glass). In biology, adhesion also means cells binding to each other or to the matrix.

Equation: h = 2γcosθ / (ρgr), where h is the capillary rise (m), γ is surface tension (N/m), θ is the contact angle (degrees or rad), ρ is liquid density (kg/m³), g is 9.8 m/s², and r is tube radius (m). Strong adhesion gives θ < 90° and a rise. Weak adhesion gives θ > 90° and a depression.

On the MCAT: Passages use capillary tubes, water rising in xylem, or meniscus shape. Predict that a narrower tube lifts the liquid higher when adhesion dominates, and that mercury is pushed down instead. Drops spread on surfaces that attract them and bead up on those that do not.

Common confusion: Surface tension comes from cohesion, not adhesion. Mercury is depressed in glass because its cohesion exceeds its adhesion to glass, not because adhesion is absent. Compare the two forces to decide.