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12 Reasons Why Contact Angles Change Over Time
When we measure contact angle, we expect
to deposit a drop of liquid on a surface so we can then measure the
static contact angle. We use Young's law to describe the forces of
cohesion and adhesion that result in a state of equilibrium. However,
often the contact angle does not remain static. Our purpose in this
article is to detail 12 reasons why contact angle can change over time.
1. Evaporation. In the case of water and aqueous liquids, evaporation is the single most common reason for a reduction in contact angle over time. Since most sessile drops have at least some hysteresis, there is a propensity for the three-phase line to remain static as drop volume decreases. This phenomenon can be seen with a drop of coffee left on a surface. Over time, the drop evaporates and the coffee particles ride down and come to rest on the three-phase line leaving a ring. If the drop had no hysteresis to start with, then the contact angle would remain constant as the liquid evaporates leaving a uniform circle of coffee particles rather than a ring. Some researchers use the evaporation method to determine the receding contact angle.
It's worth observing that the change in contact angle over time which results from evaporation is also a function of drop volume. To illustrate this point, we measured the contact angle of a 1µL drop of water on a clean glass slide. The initial contact angle measured 37.8° (line 1 on log below). We then measured after one minute (line 2) and after another minute (line 3). You can see that the contact angle after two minutes dropped from 37.8° to 27.2°, or 28%. Note that the drop width remained initially static indicating strong pinning and no significant reduction in the three phase line.
We then repeated this experiment but with a 10µL drop (lines 4-6). This time the average contact angle dropped from 37.2° to only 34.6° over the same two minute time period, a drop of only 7%. Note that on the larger drop, there is also no significant change in the diameter of the three-phase line.
We conclude that as the initial drop volume decreases, the rate of reduction in contact angle as a function of time increases.
2. Absorption. In the case of porous surfaces, the reduction of contact angle over time can be accounted for by a loss of drop volume which is consumed by or absorbed into the sample. Since the rate of absorption on some porous surfaces can be slow, the reduction in contact angle can be a function of both absorption and evaporation. In order to determine how much of the reduction in drop volume is a result of absorption, follow this procedure: First, measure the change in drop volume (using DROPimage Advanced) over a fixed time period on a solid sample with approximately the same contact angle. For example, using example above, suppose that the drop volume after two minutes dropped from 10µL to 8µL. Now repeat the measurement on the porous sample. Suppose that the drop volume over the same two minutes dropped from 10µL to 5µL. The result is that approximately 60% of the loss of volume is due to absorption while the remaining 40% can be attributed to evaporation. This, of course, is an estimate.
3. During a Cassie-to-Wenzel transition, the contact angle can drop sharply quickly. In a Cassie state, the sessile drop is setting on the top of the asperities of a non-flat surface with air trapped in the voids underneath. When a wetting transition occurs to a Wenzel state, the liquid of the drop moves into the voids under the drop. Often some type of stimulus (such as vibration or change in pressure) is necessary to trigger a Cassie-to-Wenzel transition. However, sometimes it happens as a function of time. This type of transition is irreversible and almost always results in a lower contact angle after the transition occurs.
4. A phenomenon referred to as contact line friction limits the speed of dynamic wetting. In these cases, the wetting is slowed and the three-phase line increases in diameter at a measured rate. This behavior stands in contrast to the typical equilibrium wetting state explained by Young's law. Researchers have used molecular kinetic theory and continuum mechanics to explain the phenomenon. In all fairness, contact line friction is no so much a reason for slow wetting but an explanation for the speed of wetting.
5. Liquids with a high viscosity tend to wet much slower than low viscosity liquids. Glycerol, for example, which has a viscosity more similar to honey than water, takes a long time to reach equilibrium. Thus, the higher the viscosity the longer you need to wait before attempting to measure static contact angle. With a low viscosity, equilibrium is reached nearly instantly (in less than one second, for example, with water).
6. In the cases referenced above, the factors detailed generally contribute to a contact angle which is decreasing over time. However, dissolution can explain an increase in contact angle. In the case of a polymer with a liquid drop of solvent on it, the contact angle is normally very low, even zero. When the solvent comes into contact with the polymer surface, the polymer begins to dissolve which, in turn, affects the surface tension of the solvent and inhibits its wetting efficiency. As a result, the static contact angle increases.
7. Temperature can also affect contact angle. As temperature increases, the surface tension of most liquids and the surface energy of solids decrease. This can be explained by the increase in the movement of atoms on a surface as the temperature is elevated which results in a lower cohesive force. The result is a lower contact angle as the temperature rises. Temperature can also lower the viscosity of the liquid making it wet faster.
8. A chemical reaction at the interface can also cause a change in contact angle. For example, the contact angle of aluminum on silicon carbide is about 160° at temperatures up to 900° C. However, at moderately higher temperatures, the contact angle will rapidly drop to about 45-50°. This phenomenon is the result of a chemical reaction between the liquid aluminum and the ceramic surface in which aluminum carbide is formed.
9. During polymerization wettability changes. In the case of materials used to make dental impressions, for instance, the contact angle often decreases over time as the impression material sets up. Since the purpose of the impression is to accurately capture hard and soft oral tissue topography, successful methods require hydrophilic materials which exhibit excellent wetting properties that are not significantly degraded by disinfectants or time during polymerization. It should also be noted that partially polymerized materials can have monomers dissolve in the liquid which decreases the surface tension and affects the contact angle as well.
10. In some polymer surfaces, the contact angle can change as a result of the reorientation of macromolecules on the surface. This can happen as a result of the surface coming in contact with the test liquid.
11. A change in pressure in some environments can affect the contact angle. The changes in wetting properties in high pressure environments is of particular concern to researchers in the petroleum extraction industry.
12. We propose that gravity has an effect on contact angle and that its impact increases as drop volume increases. This highly controversial position is difficult to test by experimentation. That's why we're taking a ramé-hart goniometer with us on a future Virgin Galactic spaceflight.
There are other factors that can affect
contact angle over time. If you can think of any that we have not already
addressed above, please let us know so we can include them in part two
of this article at a future date.
|How To Measure Contact Angle Over Time|
Now that we've identified some of the
factors that result in a change in contact angle over time, how do you
capture this change? All current-generation ramé-hart instrument models
can capture change in contact angle over time with varying degrees of
automation. Models with
can be configured to take automatic measurements at equidistant
intervals (e.g., every second). Our more advanced models (with
Advanced) can be operated using a methods-based experiment design
tool which includes a time file. The time file can be designed (using
the time file editor) to execute measurements at any interval or change
in interval at any rate up to the speed of the camera. To capture quick
changes we offer two
high-speed upgrade kits which can double or triple the standard rate
DROPimage Advanced also has an event editor and support for the Automated Tilting Base. This allows experiments to incorporate a change in drop volume (using the Automated Dispensing System) and a change in tilt as a function of time (using the Automated Tilting Base.) We now offer (3) models which include automated dispensing and tilting out of the box without any additional accessories - Models 290, 295, and 590. Add the optional Oscillator to capture surface dilatational elasticity and viscosity. Or add the optional Environmental Chamber (to Models 295 and 590) to take measurements at elevated temperatures.
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