Contact Line Pinning Model
A contact line is the boundary, where liquid free surface meets solid substrate. In many real-world situations, this line does not slide freely along the surface but instead pins to a fixed position, remaining stationary while the shape of the liquid above it changes.
This behavior is often observed on rough, contaminated or otherwise imperfect surfaces or due to coating. A familiar example is a coffee ring stain: as a spilled coffee droplet gradually evaporates over time, the shape of the droplet on the table largely remains unchanged, resulting in a concentrated ring of residue. Similar processes are responsible for water droplets getting stuck on a windshield. Away from the natural environment, specially engineered surfaces are used in the controlled deposition of inkjet droplets and the management of liquid films in microfluidic devices. FLOW-3D includes a contact line pinning model that enables simulation of this behavior in surface-tension-dominated flows.
A liquid aluminum alloy droplet impacts a surface with two wetting conditions: a static contact angle of 40° on the gray surface and a pinned contact line on the orange surface, showing droplet distortion and jetting at the interface between the two regions.
How It Works
The contact line pinning model works in conjunction with surface tension model and is applied as a property of geometry surfaces. When a fluid contact line encounters a solid surface marked with the pinning attribute, it becomes anchored at that location.
The pinning is enforced by setting the outward velocity of the fluid at the contact point to zero, preventing the contact line from advancing along the surface. Fluid can recede from the original contact location due to evaporation. It can also roll over the pinning point if more fluid is added to the area, but the contact line cannot slide as it would at regular geometry surfaces.
Flow near a pinned contact line is a result of the balance of forces, including surface tension forces arising from the specified static contact angle, viscous forces and gravity.
The contact line pinning model can be used together with the vaporization residue model when simulating evaporative deposition, tracking the accumulation and spreading of solid residue on the substrate.
Applications
The contact line pinning model extends the surface tension and wetting capabilities of FLOW-3D to a broader range of surface-tension-dominated problems:
- Inkjet printing for predicting droplet spreading, contact line behavior, and residue deposition patterns on substrates
- Spray and evaporative cooling for modeling the behavior of droplets on heated surfaces, where contact line dynamics influence heat transfer rates
- Coating and film deposition for simulating the behavior of liquid films on engineered surfaces, where wetting control is important
- Microfluidic devices for modeling liquid sample movement in channels and chambers, where surface engineering controls wetting and pinning behavior
- Evaporative deposition and residue formation for predicting the location and concentration of solid residue left behind as liquid evaporates from a pinned droplet