Mass flow, also known as mass transfer and bulk flow, is the movement of material matter. In physics, mass flow occurs in open systems and is often measured as occurring when moving across a certain boundary characterized by its cross-sectional area and a flow rate. In engineering and biology it may also be a flow of fluids in a tube or vessel of a certain diameter. A bulk transfer of particles of matter in a characterised type of flow is also known as bulk flow
Examples include, blood circulation, transport of water in xylem vessels and phloem tubes of plants. Transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via hydrogen bonding. If an air bubble forms the flow will be stopped as the column is broken and the pressure difference in the vessel cannot be transmitted; this is called an embolism. Once these embolisms are nucleated, the remaining water in the capillaries begins to turn to water vapor. Plants have physiological mechanisms to reestablish the capillary action within their cells. The "snapping" can be heard, and this sound can be used to measure the rate of cavitation within a plant.