Stability of thin liquid films subjected to ultrasonic vibration and characteristics of the resulting thin solid films

- Stability of thin liquid films of a solution subjected to ultrasonic vibration is studied.
- An explicit expression is obtained to predict the film stability/instability.
- Experiments are performed to validate the model.
- It is shown that the vertical vibration tends to destabilize the film.
- In practice, imposed ultrasonic vibration improves the nanostructure of the resulting thin solid film.
- Ultrasonic vibration creates a mild mixing motion without a strong destablizing effect.

Thin solid films have ubiquitous presence in many existing and emerging technologies. Solution-processed thin solid films may be fabricated by casting a thin liquid film followed by a drying step. It has been shown that by imposing a low-amplitude ultrasonic vibration on the substrate, the physical and structural characteristics of the resulting thin solid films is improved, significantly. Therefore, in this study, to investigate and rationalize the aforementioned findings, the evolution and stability of thin and ultrathin films of Newtonian liquid solutions, subjected to vertical and horizontal ultrasonic vibration is studied, using the long-wave approximation and negligible inertia forces. An explicit criterion is obtained for the film instability. Consistent with established theories, it is found that the vertical vibration tends to destabilize the thin liquid film, although for ultrasonic vibrations with low-amplitude, the term contributing to the perturbation growth rate decays rapidly with time and the film may remain stable. However, the vertical ultrasonic vibration is found as a significant destabilizing force, if the film thickness is near a critical value in which case the destabilizing van der Waals and stabilizing gravity and surface tension forces balance one another. To validate the model, experiments on thin liquid films of dilute polymeric solutions are performed. It is found that while imposing ultrasonic vibration may potentially destabilize and breakup the thin film, imposing a low-power vibration can significantly improve the homogeneity, electrical properties, and uniformity of the film, whereas a large-amplitude vibration may have a detrimental effect, because of excessive mixing and agitation of the liquid film or cracking of the resulting thin solid film.