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Why can an amorphous solid resist shear stress while a liquid cannot?
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An amorphous solid can resist shear stress because its atoms are not mobile, whereas the atoms in a liquid are free to wander throughout the material.1
On an atomic level, the distinction between these states of matter is defined by the nature of atomic motion:
- In an amorphous solid: Atoms are not mobile. Each atom oscillates rapidly about a fixed point in space, which serves as a time-averaged center of gravity. This creates a durable, albeit noncrystalline, atomic-scale structure that provides elastic stiffness against shear stress. When external forces are applied, the solid may distort, but it will return to its original shape once the forces are removed, provided the elastic limit has not been exceeded.1
- In a liquid: Atoms are mobile and continually wander throughout the material, meaning there is no enduring arrangement of atoms to maintain a shape. Consequently, a liquid cannot withstand shear stress and will flow under the action of an external force.1
While amorphous solids lack the long-range order found in crystalline solids, they possess sufficient short-range order and structural stability to maintain a definite shape, unlike liquids.12