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Researchers discovered that applying a laminar gas flow opposite to the direction of a plate being withdrawn from a liquid bath can significantly increase the critical velocity needed for liquid film entrainment. Through computational simulations and theoretical analysis, they derived mathematical expressions showing how the opposing gas flow creates interfacial shear stress that thins the liquid film and slows the contact line velocity until entrainment stops entirely. The study demonstrates that the velocity difference between the plate and contact line depends only on the applied shear stress, not the plate velocity itself.
Why it matters
This finding has practical implications for industrial coating processes where controlling film thickness and preventing unwanted liquid entrainment is critical, such as in manufacturing, printing, and surface treatment applications. The ability to arrest film entrainment through controlled gas flow could improve process efficiency and reduce material waste.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
Abstract: A plate withdrawn from a bath of partially wetting liquid entrains a film above a critical velocity. Here, we show that this critical threshold can be substantially increased by the presence of a laminar gas flow opposed to the plate velocity that exerts a tangential shear stress on the liquid-gas interface. Using two-phase immiscible Navier-Stokes simulations and confining the gas to a narrow channel, we derive a closed-form expression for the interfacial stress, which combined with lubrication theory relates the thickness of the film and the contact line velocity to the applied shear stress. As the gas velocity is increased, the film thins and the contact line velocity slows down, until the imposed shear overcomes the drag of the moving plate and entrainment is arrested. As in the quiescent case, the velocity mismatch between plate and contact line is independent of the plate velocity and is set by the applied shear alone. We find that one set of varying gas flow simulations, with constant critical plate velocity, therefore determines the critical gas flow for every smaller plate velocity.
Source: Shear-driven arrest of dip-coating film entrainment