Forced dynamic dewetting of structured surfaces: Influence of surfactants

We analyze the dewetting of printing plates for gravure printing with well-defined gravure cells. The printing plates were mounted on a rotating horizontal cylinder that is half immersed in an aqueous solution of the anionic surfactant sodium 1-decanesulfonate. The gravure plates and the presence of surfactants serve as one example of a real-world dewetting situation. When rotating the cylinder, a liquid meniscus was partially drawn out of the liquid forming a dynamic contact angle at the contact line. The dynamic contact angle is decreased on a structured surface as compared to a smooth one. This is due to contact line pinning at the borders of the gravure cells. Additionally, surfactants tend to decrease the dynamic receding contact angle. We consider the interplay between these two effects. We compare the height differences of the meniscus on the structured and unstructured area as a function of dewetting speeds. The height difference increases with increasing dewetting speed. With increasing size of the gravure cells, this height difference and the induced changes in the dynamic contact angle increased. By adding surfactant, the height difference and the changes in the contact angle for the same surface decreased. We further note that although the liquid dewets the printing plates, some liquid is always left in the gravure cell. At high enough surfactant concentrations or high enough dewetting speed, the dynamic contact angles in the structured surface approach those in flat surfaces. We conclude that surfactant reduces the influence of surface structure on dynamic dewetting.

Figure 1

(a) Optical 3D surface microscopy image of the structured surface B-25. (b) Sketch of the printing plates with two structured areas surrounded by unstructured surface. The orientation of the structured areas on the printing plate is also visible in Fig. 2.

Figure 2

(a) Photograph of the rotating drum setup. (b) Photograph of the drum with a printing plate mounted on the outer curved surface of the drum. The structured areas on the printing plate are visible by their stronger scattering of light, as indicated by the arrows. (c) Sketch of the experimental setup. Camera A is used for the height difference measurements, while camera B is used for the contact angle measurements.

Figure 3

Image of the contact line. The red dashed line marks the contact line on the structured surface and the blue solid line marks that on the unstructured surface. The difference between these is defined as the height difference $\mathrm{\Delta }h$ (black arrow). The yellow arrow indicates the direction of motion of the surface.

Figure 4

Receding contact angle oscillation of a deflating drop on the B-25 surface. During deflation of the drop, the contact line moves to the left. (a) Contact angle on the flat part of the structured surface. (b) Due to pinning at the corner of the gravure cell, the contact angle changes while the drop moves over a gravure cell. The lines show the variation of the contact angle. (c) The contact angle variation during the movement of the contact line over the structured surface. Oscillations are due to pinning at the gravure cells. The contact angle varies by more than 15°. The dashed line is the contact angle on the smooth surface. The measurement error can be estimated to approximately 3°, as indicated in the plot.

Figure 5

Measured height difference $\mathrm{\Delta }h$ of the contact line for water between neighboring structured and unstructured parts of the printing plate (see Fig. 3 for the definition of $\mathrm{\Delta }h$). The experiments were performed on a printing plate with 262 $\mu \mathrm{m}$ × 262 $\mu \mathrm{m}$ distances between the gravure cells. The red diamonds represent the structured area with the deepest gravures. The dashed lines are guides to the eye for the data with smaller gravures (compare Table 1). The error bars represent the standard deviation of ten independent measurements.

Figure 6

(a) Measured receding contact angles (opened squares) of water on different surfaces. To calculate the contact angle on the structured part of the plate, Eqs. (1) and (2) were used. Here, we used a surface tension of 72 mN/m and a density of $998.2\mathrm{kg}/{\mathrm{m}}^3$. For better visibility, the error bars are only shown for the unstructured surface. For the structured surfaces, the error is ±6°. (b) Height of the meniscus over the unperturbed liquid level. The $x$-axis is broken to sketch the whole meniscus shape.

Figure 7

(a) Measured height difference of the contact line on the structured and unstructured part on the printing plate B-25 for different surfactant concentrations. The concentration varies from 0% to 45% of the critical micelle concentration of S-1DeS. The red dotted line illustrates the height difference plateau $\mathrm{\Delta }{h}_{\mathrm{pl}}$. The error bars are obtained from ten independent measurements. (b) Change in the height difference of the contact line on different structured plates for different concentrations at a velocity of 15 mm/s. The lines are guides to the eye. (c) Calculated contact angle based on Eqs. (1) and (2) on the surface B-25. The error bars are similar for all concentrations. (d) Comparison of the change in the contact angle due to the addition of S-1DeS on the smooth part of the printing plate (red circles) and the structured surface B-25 (green rectangles) with the data published in Ref. [24] (black squares). The lines are guides for the eye.

Figure 8

(a) Sketch of the process close to the three-phase contact line in the presence of surfactants. The size of the surfactant molecules is not to scale (see the main text for details). Part (b) illustrates the definition of the quantities used in the scaling arguments.

Figure 9

Measured height difference of the plateau $\mathrm{\Delta }{h}_{\mathrm{pl}}$ [see Fig. 7] for all different kinds of surfaces employed. The height difference is plotted over the area of one gravure cell for both distances between the gravure cells. With increasing surfactant concentration, the influence of the structure decreases. The bars represent the standard deviation of the data points in the plateau. The dashed lines are guides to the eye.

Figure 10

Parts (a)–(c) show the emptying process of a gravure cell. While (a) defines the passage time as the time the contact line needs from the top corner of the gravure cell to depin from the bottom corner, (b) depicts the emptying for pure water, and (c) depicts the emptying for a solution with 15% CMC of S-1DeS. The first frame in (b) shows the appearance of the gravure cell at the contact line, the second one the middle of the emptying process (after 2.5 ms), and the third frame the depinning of the contact line from the bottom of the gravure cell. Part (d) presents the passage time for one gravure cell with and without surfactant for different velocities. The solid blue line indicates the passage time for a constant contact line velocity.

Figure 11

IR image of a moving drop. The left side shows an unstructured surface, the right side the structured surface B-25. Darker regions emit less IR radiation (e.g., they are colder or reflect less IR radiation) than brighter regions.