Linear feedback control of liquid film on moving substrate via free-surface stresses

Abstract

Liquid films on moving substrates are used in dip-coating processes to form uniform protective layers. Controlling free-surface waves is essential due to the film's inherent linear instability. Therefore, we develop a linear feedback controller to regulate the film toward a desired flat state by modulating the free-surface shear and pressure, with feedback gains derived analytically from linearised equations. Control performance is assessed for finite-amplitude waves using a Weighted Integral Boundary-Layer (WIBL) model at reduced Reynolds number $\delta = 8$. We identify parameter regimes in which pressure feedback is linearly destabilising while shear is stabilising, and vice versa, with the control mechanisms determined by the balance between the kinematic and dynamic wave velocities. Both stabilising and destabilising combinations of feedback coefficients can drive finite-amplitude waves toward the flat state $\bar{h}=1.1$ in finite time. In pressure-unstable regimes, the control induces a limit-cycle behaviour, in which long waves decay slowly due to the interplay between thickness and slope terms. The travelling-wave solution, although it decays slowly, moves against gravity, whereas other combinations reduce the wave amplitude in the direction of uncontrolled propagation. These results provide a foundation for higher-Reynolds-number studies and the design of industrially feasible actuator layouts.