Valve incorporating temporary phase change material

A method of selectively blocking or opening a passage using a granular gel that is a thixotropic material, the method comprising:

• blocking the passage using the granular gel, wherein the granular gel comprises a cross-linked polymer network that is solvated in a solvent and wherein the blocking comprises:
• operating at least one pump to dispense the granular gel into the passage by applying a first pressure to the granular gel that is above a yield stress of the granular gel and causes a first phase change in the granular gel;
• and
• after the granular gel is in the passage, operating the at least one pump to impose a second pressure on the passage that is below the yield stress of the granular gel and causing a second phase change in the granular gel.