Experiments on the spreading of droplets on rotating plates and their stability

• Applicant: Professor Dr. Peter Ehrhard
• Subject Area: Fluid Mechanics ; Coating and Surface Technology ; Mechanical Process Engineering
• Term: from 2011 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 194067256

The spreading of liquids on rotating substrates, on the one hand, is the basis of the spin-coating process. This process is often engaged, if a solid coating of plane substrates is needed. Examples include semi-conductors, solar panels, or liquid-crystal displays (LCDs), where in all cases numerous solid layers of defined electrical or optical properties have to be added. In most cases, this coating applies initially a homogeneous liquid layer, which transform due to solidification or evaporation of the solvent into a solid layer. On the other hand, the modelling of such coating flows and their stability presents a scientific challenge, since at least one moving contact line is involved, a region where the fluid-layer thickness becomes very small. Hence, continuum mechanics is expected to touch its limit of validity within those regions. Even a secondary moving contact line may occur, if the liquid layer ruptures at the centre of rotation and creates a dewetting front.During the last decades, the theoretical modelling of this process has significantly advanced. Particularly, adequate models are available which capture the rotationally-symmetric spreading base flow and its (linear) stability against small disturbances which are periodic in the circumferential direction. Nevertheless, careful experiments on this process remain rare, which makes it hard to validate the theoretical models. The few published experiments focus on liquid contours during continuous liquid pouring at the rotation centre, on liquid contours during spreading across non-plane substrates, and on the finger instability of the contact line. Our proposed experiments aim to provide a broad data basis for the spreading of perfectly- and partially-wetting, Newtonian liquids on plane rotating substrates. We shall capture the dynamics of the (mean) drop radius, the speed of the contact line, and particularly contours of spreading droplets in time. This allows also to infer the history of the contact angle. Moreover, the rupture of the liquid film at the centre of rotation and the dynamic behavior of the corresponding dewetting front is an important aspect of our investigations. Finally, the systematic investigation of the stability of the wetting front is within our focus.

• DFG Programme: Research Grants
• Participating Person: Dr.-Ing. Konrad Boettcher