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Failure of thermocapillary-driven permanent nonwetting droplets
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Image of FIG. 1.
FIG. 1.

Sketch of thermocapillary convection near the contact point. The flow of the surrounding gas is maintained as long as the temperature difference is maintained.

Image of FIG. 2.
FIG. 2.

High speed sequences of (a) film and (b) pinning failure. Time intervals between the images are 1/2000 and 1/200 s, respectively.

Image of FIG. 3.
FIG. 3.

[(a)–(c)] Deformation of a permanent nonwetting droplet with an increasing displacement against a cold plate. The increase in capillary pressure associated with the droplet deformation is balanced by the pressure field within the lubricating film flow driven by thermocapillary convection. The reflection of the droplet can be seen on the glass surface near the top of each image.

Image of FIG. 4.
FIG. 4.

Experimental setup schematic. Interferometry is used to detect the point of initial “contact,” as well as monitor the shape of the interface during the loading.

Image of FIG. 5.
FIG. 5.

Typical procedure employed by SE simulations to calculate the load on a droplet. (a) Polygon subjected to constraints and boundary conditions. (b) Shape of a sessile (or pendant) droplet is obtained by minimizing the energy of the polygon in (a). (c) Deformed droplet, squeezed by a constraint plane that prevents penetration of the liquid surfaces. The case shown here is for using the properties of 5 cS silicone oil.

Image of FIG. 6.
FIG. 6.

Load curves, computed by SE, for droplets pinned to a 3 mm diameter pedestal with the properties of 5 cS silicone oil. The force applied on the droplet is computed from the change in the system energy as a function of relative squeezing introduced on the droplet. The legend indicates the droplet volume in .

Image of FIG. 7.
FIG. 7.

Change in the system energy as a function of relative displacement of a droplet against a nonwetting plane. The onset of shape instability occurs at A, where the energy of the droplet drops abruptly, resulting in the shape as shown in B. This curve is obtained from a droplet with the properties of 5 cS silicone oil with the volume of .

Image of FIG. 8.
FIG. 8.

Load corresponding to film failure as a function of volume for (a) 5 cS and (b) 10 cS silicone oil droplets deposited on a 3 mm pedestal. The legends indicate the temperature difference, .

Image of FIG. 9.
FIG. 9.

Sequence of disturbance growth just prior to film failure. The quarter of the dimple contour (seen as a thin arc crossing from top-left to bottom-right corner) is shown. (a) The onset of disturbance growth at . The nonaxisymmetric disturbance grows from a spot (indicated by the arrow) on the interface. [(b) and (c)] The disturbance propagates in the azimuthal direction, shown by the arrow. The images are taken at (b) and (c) 2 s. Wetting takes place shortly after (c).

Image of FIG. 10.
FIG. 10.

(a) Coordinate system used for the image analysis. (b) Height variation around the contact region obtained from the images in Fig. 9. The disturbance is initiated around and spreads in the azimuthal direction. As a result, the locations of the liquid-gas are displaced in the range of .

Image of FIG. 11.
FIG. 11.

Pinning failure load as a function of volume for (a) 5 cS and (b) 10 cS silicone oil droplets deposited on a 3 mm pedestal. The experimental data are compared with loads computed by SE at the onset of shape instability.

Image of FIG. 12.
FIG. 12.

Images of unloaded droplets with (a) and (b) . Assuming that there exists a threshold contact angle for pinning failure, the droplet in (a) must experience a larger change in the pinning angle than the droplet in (b) in order to reach the threshold.

Image of FIG. 13.
FIG. 13.

Comparisons of failure load obtained with two different antiwetting coatings, namely, SG and RO. The cases shown here are (a) 5 cS at and (b) 10 cS oils at . The use of RO in (a) resulted in film failure, which did not take place with SG in this condition. The failure loads obtained with RO in (b) are systematically lower than those obtained with SG. The failure modes are as indicated in the legend.

Image of FIG. 14.
FIG. 14.

(a) Interferogram of the contact region affected by an apparent solutocapillary effect. (b) Masked region of (a) where the reconstruction algorithm is applied. (c) Reconstructed surface. [(d)–(f)] Same as (a)–(c) except at a higher applied load.


Generic image for table
Table I.

Relevant properties of silicone oils.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Failure of thermocapillary-driven permanent nonwetting droplets