Non-equilibrium thermal fluctuations present as wave elements in a flow. A wave element is the wave interface between two molecule groups with different temperature; it is generated by density difference which results from temperature difference. Tiny temperature differences always exist everywhere in a fluid. When the fluid is in motion, wave elements are generated among molecule groups. Wave motion and Brownian motion may be the two basic forms of motion of molecules in flow. Brownian motion is controlled by temperature. Wave elements are caused by temperature differences and the motion of the fluid. Wave motion maybe the physical mechanism of convective heat transfer. Non-equilibrium thermal fluctuations exist everywhere among molecule groups in a flow. The theoretical analysis presents that a wave element presents oscillatory behavior along the space and time dimensions simultaneously. The experimental evidence for wave elements can not be directly established at present scientific testing capability because the temperature difference of two molecule groups adjoining to each other in a flow is very small. A series of "enlarged size" experiments of fouling to show the behaviors of wave elements by tracing the movement of molecules are conducted. The experimental study of fouling presents that oscillatory interface along the space and time dimensions simultaneously exists between two densities due to motion of the fluids. The experimental and theoretical analyses are supported to each other.
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