In a U-tube experiment, water moves from:
A classical U-tube experiment visualizes osmosis by dividing tube into two arms with a membrane impermeable to solute like sodium chloride but permeable to water via narrow pores or aquaporin analogues. Each arm filled with aqueous solution of different concentration. Water activity and chemical potential are inversely related to solute concentration; lower solute means more free water molecules with higher escaping tendency. Consequently random thermal motion results in greater diffusive flux of water from low solute concentration side where water concentration high toward high solute side where water concentration low. Solute molecules cannot follow due to size or polarity exclusion. Over time concentrated arm level rises while dilute arm falls, generating hydrostatic pressure difference that eventually balances osmotic driving force at equilibrium. No active transport or energy input is involved beyond existing gradient. This net flux demonstrates that water moves opposite to solute gradient. Hence water movement occurs from region of lower solute concentration to region of higher solute concentration until potentials equalize.
Ref: Lodish et al., Molecular Cell Biology, Chapter: Osmosis and U-Tube Demonstration of Water Movement.