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Author: Ali Kosari Mehr
Being a thick-film deposition method, dip coating is utilized for the deposition of uniform coatings by immersion and subsequent removal of a sample from a liquid. To ensure the uniformity of coatings, the inks to be used in this process are selected to be dilute suspensions of powders (0.01–0.10 g/ml). The method’s thickness capability, ink viscosity range, and powder loading are 1-50 μm, <0.01 Pa-s, and 0.5–5 vol%, respectively.
During the process, one should ensure the absence of the particles sedimented out of solution, considering that the process is fairly slow. Moreover, the size of the samples to be coated can be a determining factor in this process since large samples have to be immersed in large tanks of inks, resulting in this method not being adequate for the deposition of expensive/hazardous materials.
The deposition process can be divided into four phases: Immersion (a substrate goes into liquid), deposition (the substrate is now completely immersed), drainage (the substrate is removed from the liquid), and evaporation (the coated substrate dries). However, It must be noted that evaporation happens in all the above-mentioned phases.
In the drainage phase, the balance between gravity aiming at removing the ink from the substrate and the ink’s viscous drag aiming at keeping the formed film on the substrate defines a parameter called the thickness of the wet film. Provided the meniscus effect is marginal (i.e., high withdrawal velocities), the thickness (h) can be shown by the following expression:
Where νw, ρi, and ηi are withdrawal velocity, ink density, and liquid viscosity, respectively. Provided the withdrawal velocity and liquid viscosity are low, the meniscus surface tension influences the wet thickness:
Where γ1v is surface free energy at the vapor-liquid interface. However, in either case, thicker films can form as surface energies and liquid densities decrease and withdrawal velocities and liquid viscosity increase.
Finally, it must be noted that the final thickness of the films – after evaporation of the wet films – is dependent on the inks’ properties, as a result of which a considerable reduction in the final thickness of the dried films is expected owing to the low powder loading in this process. Furthermore, sufficient time for particle rearrangement throughout evaporation – as a result of careful withdrawal – can cause the withdrawn wet films to be very well packed. The rationale behind this phenomenon is that individual particles should be able to gather together on the surface without obstruction (a reason for the low powder loading requirement in this process). It is noteworthy that large-scale particle clumping can be observed if withdrawal velocities are very high.
