Fluid Velocity Gradient
Mechanical rate variations within moving liquid suspensions control the dispersion, deflocculation, and spatial orientation of suspended cellulosic fibers. Across the forming section of a paper machine, hydrodynamic shear emerges wherever adjacent layers of stock travel at differing linear velocities or encounter stationary drainage elements. This physical gradient counters the natural tendency of cellulose fibers to aggregate into coherent flocs prior to drainage.
Refining stages, approach flow manifolds, headbox slices, and wire tables each apply targeted shear magnitudes to condition the suspension. The mechanism ceases to govern fiber motion once the web structure achieves mechanical immobilization through progressive dewatering past the dry line.
Dispersion Mechanism
Pumping stock through pressurized headbox nozzles accelerates the suspension and creates steep velocity boundaries near internal boundary surfaces. Within this turbulent stream, hydrodynamic shear generates localized rotational moments that twist and pull entangled fibers apart into separate filaments. Controlled turbulence keeps fibers dispersed uniformly across the slurry without generating sustained vortices that cause macro-scale basis weight streaking.
Lower shear regimes allow secondary flocculation, producing cloudy sheet formation and localized caliper instability across the finished reel. Excessive shear degrades long fibers, strips external fibrils from cell walls, and reduces overall mechanical retention on the forming fabric by washing mineral fillers through the wire.
Structural Anisotropy
Differential velocity between the discharging slurry jet and the moving forming fabric establishes a dominant directional shear field at the moment of initial drainage. Operating with mismatched jet and wire velocities forces flexible fibers to rotate and align parallel to the direction of web travel. High hydrodynamic shear at this contact interface amplifies the ratio of machine-direction to cross-direction tensile stiffness, producing an anisotropic sheet structure.
Balanced configurations reduce orientation intensity, generating square sheets desired for high-speed folding cartons and multidirectional converting applications. Web break frequency on high-speed printing presses increases when uncontrolled shear creates localized orientation streaks that fail under cross-web tension.
