The spreading of liquid films is involved in many coating processes, e.g. in the spin-coating process. To achieve a high quality of the coating, the spreading liquid layer should be thin and homogeneous. Instabilities at the wetting front may lead to an inhomogeneous thickness of the coating layer and to uncoated areas. In this article the spreading of perfectlywetting silicone oil droplets with viscosity of 100 mPa s on rotating glass plates is discussed. A Schlieren system is set up to observe the wetted area and a traversed chromatic confocal distance sensor is used to measure the contour of the droplet. The experimental data are presented and compared to an analytical model which is derived from lubrication theory and valid for thin liquid layers. Basic stateThe following dimensionless groups capture the ratios of various forces,namely the Bond number G, the capillary number C and the centrifugal number Φ. G reflects the ratio of gravitational and capillary forces, C the ratio of viscous and capillary forces and Φ the ratio of centrifugal and capillary forces. Herein, the properties of the liquid are ̺, µ, σ, namely density, dynamic viscosity and surface tension. The initial data for the droplet are r 0 , r t 0 , θ 0 , namely the initial radius, contact-line speed and apparent contact angle. g is the gravitational acceleration and Ω the angular frequency. An analytical model for the dynamic spreading of axisymmetric droplets is described by Ehrhard and Davis [1]. By invoking the lubrication approximation, an analytical solution can be derived in the limit of C → 0. Boettcher and Ehrhard [2] extend this model by including the rotation of the substrate and perform a linear stability analysis by introducing non-axisymmetric disturbances. The aim of the present experiments is to validate this model. The capillary number in our experiments is in the range C ≤ 0.05.To gain information about contact angle, radius and contact-line speed, the droplet needs to be observed in time. The measurement of the contour of the droplet is realized by using a traversed chromatic confocal distance sensor, which measures the distance from the sensor to the liquid/gas interface. By subtracting the initially-measured level of the rotating plate, the contour of the droplet can be inferred. The chromatic confocal distance sensor is mounted on a crossbar and is traversed radially across the rotating droplet through its center. The sensor gathers information along that line at up to 2 kHz. The radius and contour can be directly derived from the sensor data. The contact angle is calculated by fitting a polynomial function to the discrete points of the contour of the droplet. The derivative of this function with respect to r at the edge of the droplet allows to infer the contact angle.2 Spreading at different angular frequencies Figure 1a shows the evolution of two different droplets, measured by the chromatic confocal sensor. On the left side, the spreading of a droplet on a non-rotating plate is shown. On the right side, a droplet with a...
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