ramé-hart Newsletter

                  

August 2020
 

In the meantime, there's one big drop right in front of your eyes and on the other side of the windshield. Let's focus on that drop for a minute. There are lot of forces acting on it. As you speed up (V), the drop wants to ride up the glass. The car's movement forward is similar to a strong wind coming directly into a stationary car. On the other hand, gravity (v) will want the drop to roll forward and down the windshield toward the hood depending on the tilt angle of the windshield (θ).

Then there's the surface tension of water - which is high. Think of an invisible rubber sheet (shown in green in the graphic below) that surrounds the drop (in blue) making it want to become a ball. In fact, that's what it would do if there were no other forces present. 

But the surface energy of the windshield - which is also relatively high - is pulling the drop toward the glass. And then there's gravity which is pulling it toward the earth. Chances are the drop is also dancing around due to the erratic changes in the wind, car vibration, car speed, and other external forces. It's a small miracle that it stays put - pinned to the windshield.

You arrive home, pull into the garage and shut the door. We've now eliminated most of the forces caused by moving, the wind, and car vibrations. And that's a good thing - since this is not a physics class. Our rain drop is a static sessile drop that we can more carefully analyze.

Now if you get out of your car - and do it quickly before the drop evaporates - you will see that the contact angle on the drop is greatest on the downhill side and smallest on the uphill side. That's gravity at work. Ideally, if you could tilt your windshield forward to increase the tilt angle, you will find that the downhill contact angle continues to increase while the uphill contact angle continues to decrease. Yea, that's hard to do unless you have some kind of combat vehicle with a drop-down windshield. Anyway, we refer to the largest possible contact angle as "a" or advancing contact angle while the smallest contact angle is "r", the receding contact angle. If there are still other drops on your windshield, you would also notice that the larger drops will roll off sooner than the small drops as you increase the tilt angle. In short, the roll-off angle is sensitive to drop volume.

As you can see, there is a spectrum of potential contact angles depending on the tilt angle. If we focus only on the largest (or advancing) contact angle and the smallest (or receding) contact angle, we can arrive at the contact angle hysteresis (CAH) - which is the difference between the two.

Assuming that CAH is important, let's pause to review four common ways to measure it:

• The tilting base method detailed above.¹ You can measure it on your car window but for more accurate results we recommend a ramé-instrument equipped with the Automated Tilting Base.
• The add/remove volume method. A needle is embedded in a drop and the contact angle is measured as the volume is increased. At the point prior to wetting, or an increase in the three-phase line, the maximum advancing contact angle is measured. Volume is then retracted and contact angle is measured until dewetting (a decrease in the three-phase line) occurs. The final smallest contact angle prior to dewetting is the receding contact angle.² We recommend a ramé-hart instrument equipped with the Automated Dispensing System to perform this rather automatically.
• A drop is measured over time as the drop evaporates. The initial drop is the advancing contact angle. The final contact angle prior to dewetting is captured and used as the receding contact angle.
• A captive bubble can be produced on the underside of a submerged substrate. The contact angle can be measured with air added and then removed from the drop. 

It's worth noting that different methods produce varying results. Thus, it's not wise to mix and match methods if you're trying to compare one solid/liquid set to another. Once we've determined the CAH, let's review what it's telling us. For starters, if the CAH is small, this would indicate that the surface is relatively homogenous in terms of chemical and structural composition. The exception to this rule would be nanosurfaces which result in a Cassie state. In that case, the CAH can be low while roughness is high.

Non-Cassie state solid/liquid combinations typically produce larger CAH. Even when the surface is ideal or close to it, there is still some CAH. Take silicon wafers, for example. They are perfectly flat, rigid, and smooth - that is, nearly ideal. Despite this, our customers who measure CAH using the tilting base method on wafers capture CAH values in the 6-12° neighborhood. This can be explained by pinning forces, a topic for another newsletter. Briefly however, when the three-phase line is pinned, volume can be added or removed, or the solid can be tilted, resulting in an observable CAH - even if the surface is ideal or nearly so. Pinning, by the way, is what keeps the raindrop from rolling off the tilted windshield. 

For less-than-ideal surfaces (which are the most common type) that entertain drops in the Wenzel state (which is the most common state), CAH reflects heterogeneity in terms of chemical and structural composition. Static contact angles can signal CAH when the L and R contact angle are far apart in value. The difference should be used as an indicator of CAH but not used for calculating it.

Back to our windshield study, you have more to think about now the next time a storm creeps up on you and pelts your car with raindrops on your way home. If you like to game the system, try a little Rain-X. It contains polydimethylsiloxane, a hydrophobic silicone polymer, which increases contact angle, decreases CAH, and promotes low roll-off angle. The raindrops will blow or roll off before you manage to park your car and get out to study them - which is a good thing - it leaves you more time to play with the kids.

Bullets

• The advancing contact angle is the largest possible contact angle for a given solid/liquid combination.
• The receding contact angle is the smallest possible contact angle for a given solid/liquid combination.
• The roll-off angle is sensitive to drop volume. As drop volume increases, the roll-off angle decreases.
• The contact angle hysteresis (CAH) is the difference between the advancing and receding contact angles. 
• Advancing and receding contact angles can be measured by the tilting plate method, the add/remove volume method, evaporation, and captive bubble.
• Even ideal surfaces can exhibit CAH due to pinning forces.
• High CAH values reflect heterogeneity in terms of both chemical and structural composition.
• Rain-X is a hydrophobic coating that promotes decreased surface energy and lower CAH on a windshield.

Notes

¹ This video shows how DROPimage Advanced with the ramé-hart Automated Tilting Base can be used to measure advancing and receding contact angle using the tilting base method.
² This video shows the add/remove volume method with the ramé-hart Automated Dispensing System.
 

If the article above got you excited about measuring advancing and receding contact angles, you'll find the windshield method produces rather poor results. For precise results, we recommend a ramé-hart instrument equipped with DROPimage Advanced software and the Automated Tilting Base option. The Automated Tilting Base offers a full 180° of rotational freedom from -90° to +90°. You can control the speed, tilt angle, and step angle. However, the real power of tilting comes when the tilt is embedded in a methods-based experiment as shown in this video.

ramé-hart Automated Tilting Base p/n 100-25-A (also shown is the Automated Dispensing System and Model 260 installed on the Automated Tilting Base)

Model 290 and Model 590 include the Automated Tilting Base but it is available as an option for any other model equipped with DROPimage Advanced.

If you have any questions regarding the Automated Tilting Base or would like a quotation, don't hesitate to contact us.

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
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