Cleaning requires dislodging a soil and carrying it away from a surface. For this to happen, the cleaning or rinse fluid must energetically access the surface. However, a boundary layer of liquid with reduced flow velocities can impede cleaning. A similar effect occurs in flowing waterways, like a river or stream when flow velocities are lower near the shallow edge than in the deeper middle. In this case, debris tends to pile up on the shore. What causes the boundary layer? How does it prevent effective cleaning? What can you do about it?
How does a cleaning agent remove a contaminant from a surface? One mechanism is solvency, when the chemical forces of the agent, in essence, unhook the contaminant from the surface. A second mechanism is mechanical motion through which contaminants can be knocked off by momentum. The mechanical motion of fluid flow also carries dislodged contamination away from the surface. With both of these mechanisms, the boundary layer can inhibit cleaning.
The boundary layer is a region of reduced fluid flow velocity near the surface. When the fluid velocity decreases, it cannot effectively bring new cleaning agent to dissolve contaminants or to carry them away when dislodged. In addition, the reduced velocity means that there may be insufficient energy to knock contaminants from the surface.
What causes the boundary layer? Without getting involved with details of physics, here is what’s happening. Due to its mass, a molecule of a fluid possesses inertia, a resistance to change in velocity. The molecule can also be attracted to adjacent molecules. The molecules right at the solid surface are slowed to a stop by this attraction. Those just above, collide and are slowed by the attractive, viscous, effect, but because of inertia, aren’t completely stopped. Those above are slowed a bit less. As a result, a layer of fluid, the boundary layer, with reduced flow velocity occurs near the surface (1,2) as indicated in the figure below. The thickness of this layer depends on the ratio of inertial forces to viscous forces. Low viscosity fluids have a thinner boundary layer.
How can you overcome boundary layer effects? One way is to direct the cleaning agent or rinse fluid toward the surface, such as by a forced spray. A similar effect can be obtained during ultrasonic cleaning when the shock wave from a collapsing cavitation “bubble” creates a jet of liquid directed to the surface. However, these effects can be difficult or impossible to obtain in narrow spaces such as in tubes or cannula. One promising method to overcome this is the flushing and refilling that occurs in a cyclic vacuum process such as the Cyclic Nucleation process (CNp) (3,4).
When evaluating the effectiveness of a liquid cleaning process, consider what happens right at the surface. Is the liquid reaching the surface? Is there sufficient motion to dislodge and remove contaminants and carry them away from the surface?