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April 2022 |
| Pinning |
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Our aim this month is to offer a clear
explanation of the phenomenon which is referred to as pinning, more
specifically the pinning of a sessile drop at the three-phase line.
Think of this way: a drop of liquid is sitting on a surface; the
surface is tilted; the drop stays put. It's pinning that keeps the
drop where it's at. As a result, a variety of observable contact
angles are possible which ultimately produces the the contact angle
hysteresis, or in other words, the difference between the maximum or
advancing contact angle and the smallest or receding contact angle.
There is a direct relationship between contact angle hysteresis and the forces that explain pinning. Consider the following experiment: We start with a glass slide. We then clean it thoroughly using acetone and deposit an 8 µl water drop on the surface. We then tilt the sample using our ramé-hart Automated Tilting Base and discover that the drop releases and rolls off at 30°. Note that glass is a high surface energy solid and while it's perfectly smooth and chemically homogenous, there is a measurable contact angle hysteresis. Next we take that same glass slide to our sandblaster and we blast the surface leaving it etched. After thoroughly cleaning it, we return the tilting base and we observe that an 8 µl water drop stays pinned to the surface even when the tilt angle increases to 90°. We also observe an increase in the static contact angle and significant increase in the contact angle hysteresis. One obvious conclusion we draw is that when the surface roughness increases on hydrophilic surfaces, so too does the contact angle, and the contact angle hysteresis as well as the roll-off angle. In other words, increased roughness increases pinning. The other condition that can result in increased pinning is lack of chemical homogeneity. If the wetting property varies from one spot to the next on a surface, the energy required to wet and dewet increases. In other words, more energy (e.g., tilt) is required to promote slip-stick movement. But, what if you are working with an ideal surface? Or one that is nearly so? Extrand, et al, discovered that even silicon wafers - which are about the most structurally and chemically homogenous materials known to man - will exhibit a contact angle hysteresis.1 This behavior is explained by pinning which is nothing more than a static resistance to shear located uniquely at the three-phase line. In short, the intermolecular forces that occur between the solid phase and the liquid phase at the three-phase line result in pinning and explain how the contact angle hysteresis can be greater than zero even on ideal surfaces or those that are nearly so. Pinning also contributes to the explanation for the coffee ring effect. When coffee or any other particle-laden liquid is left on a surface to evaporate, the particulates migrate toward the pinned three-phase line which explains why the perimeter of the coffee stain is noticeably darker than the interior portions. The surface area remains static during evaporation due to pinning at the three-phase line. Another way to measure pinning force is to perform a depinning study. Consider this experiment: A large water drop, say 10 µl, is dispensed on test surface A. The needle is left in the drop and while volume is slowly retracted from the drop at a high resolution of 0.02 µl per step. At the same time, the contact angle and drop width are being constantly measured multiple times per second. When the drop reaches 2 µl, it's observed that the drop made eight depinning steps. That is, when approximately 1 µl of liquid is removed, the three-phase line unpins and contracts before pinning again. This depinning-pinning event repeats a total of eight times.
Now the experiment is repeated under identical circumstances but with surface B. This time, it's observed that the drop never dewets. The drop width remains static during the entire experiment as the drop is reduced from 10 µl to 2 µl. In an effort to explain the reason for the stronger pinning forces, we look at both structural differences (i.e., surface roughness) as well as differences in chemical homogeneity. Note, too, that this experiment, if you have the time, can also be accomplished via evaporation. As we strive to become more familiar with wetting behavior, we recognize the importance of observing pinning forces and their faithful correlation to contact angle hysteresis and roll-off angle. We also observe the impact that surface heterogeneity (with respect to structural and chemical composition) has on pinning behavior. Notes |
| Product of the Month: Environmental Chamber |
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The ramé-hart
Environmental Chamber (p/n 100-07) has a long history of providing a
controlled elevated temperature environment for contact angle, surface
energy, surface tension, and interfacial tension studies. The ramé-hart
Temperature Controller (p/n 100-50) can tightly achieve any
temperatures between ambient and 300°C and hold it to within 0.1°C.
Quartz windows provide optical purity and cooling ports permit a coolant
to be cycled through the base of the chamber in order to achieve a
sub-ambient temperature, or to bring the temperature down rapidly. Add a
standard ramé-hart
Quartz Cell (p/n 100-50) and
Inverted Needle (p/n 100-10-13-22) to measure interfacial tension (IFT)
at elevated temperatures. Add the
Cover with
Stage (p/n 100-09) and you're all set to measure captive bubbles and
inverted sessile drops in a liquid/liquid heated environment.
The Environmental Chamber, due to its size and weight requires Model 260, 500, 590, or 790. For smaller instruments, check out our Hot Plate and Heated Environmental Cell accessories. Please
contact
us if you have any questions regarding the ramé-hart Environmental
Chamber or any other ramé-hart product. We would be happy to answer any
question or provide an official quotation. |
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Regards,
Carl Clegg |
