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|Are 3M Post-it Notes1 the Best Sticky Notes?|
First some history: It turns out that a
chemist at 3M, Spencer Silver, came up with a pressure-sensitive adhesive
that is reusable and not too tacky or messy. He had been working on it for
years and finally patented it in 1968. For six years he beat the drum
for his invention at 3M but never really had an application in mind. It wasn't
until 1974 that another 3M employee, Art Fry, had an epiphany while singing
at church - a
bookmark for his hymnbook. In 1977 3M launched the first generation of
sticky notes called "Press 'n Peel" but no one seemed to be interested
yet. After reworking their marketing efforts 3M launched the Post-it Note
in 1980 and in the subsequent three decades, the Post-it has become one of
the greatest product success stories in the history of 3M.
How does it work? A Post-it Note has thousands of small elastomeric copolymer microspheres applied to a paper. The result is a low-tack pressure sensitive surface which can be easily applied to and removed from another surface without leaving residue. The sticky note shares some of the properties of the Gecko toe pad: it attaches strongly with minimal preload, detaches with limited effort, sticks to most any solid surface, leaves no mark, and will self-adhere. While the Gecko foot is a much more sophisticated adhesive mechanism than the sticky note, to the extent that they each share some basic properties, we anticipate that the adhesive surface on the sticky note is hydrophobic.
There are now many generic sticky notes on the market. Our question is: Is the original 3M Post-it the best sticky note on the market, or are all sticky notes the same? To answer this question, we began by comparing the contact angle on the adhesive surface of a 3M Post-it to that of a Staples brand Stickies Adhesive Note1. Since the sticky note borrows some of the adhesive characteristics of the Gecko toe, it's presumed that the higher the water contact angle is, the better the stick will be. After all, the water contact angle of Gecko setae is 160.9°. It turns out that the Post-it Note measured an average contact angle of 123.41° while the Staples Stickies note measured only 102.73° on average. In addition we noted that it took 10 seconds for the Stickies note contact angle to drop 10° while the Post-it Note only dropped 5° after five minutes. The initial observation is: the Post-it has a more Gecko-like grip with virtually no absorption into the paper backing; the Stickies note likely has less adhesive or a lower-grade adhesive which fails to prevent absorption and has a less effective stick. Based on our experience with other PSA (pressure-sensitive adhesive) products that we've tested, we conclude that the Post-it is has a superior adhesive surface - based on our observations of both contact angle and rate of absorption.
However, before handing out awards, we wish to confirm our conclusions by performing a stiction test. To do this, we stuck the Post-it and Stickies notes on the same surface (using the same amount of pressure) and then applied a downward sheer force on the tails of the notes in a direction perpendicular to the mounting surface (a shear force test). The Stickies required 1221 g to release while the Post-it required 2037 g. So the Post-it wins again requiring over 66% more shear force to release from a surface than the Stickies note.
Draw your own final conclusion. But we're going to go with 3M Post-it Notes in our office. They simply stick more like a Gecko's foot.
1 Post-it Notes is a
registered trademark of 3M. Stickies Adhesive Notes is a trademark of
|How to Measure Advancing and Receding Contact Angles Using the Automated Dispensing System|
It turns out that a lot of our customers
who already have an
Automated Dispensing System, wish to use it to measure advancing and
receding contact angles using the add and remove volume method. It turns
out to be a much easier task than anticipated by most. In the video
below (or point your browser to
http://youtu.be/1wh0VtnCIEs), we use the Automated Dispensing System
to first produce a sessile drop with a specific volume. Throughout the
experiment we leave the needle embedded in the drop which requires us to
turn on the right line option. In the Contact Angle Tool, we use the
Steps option to add volume (or step out) and measure. We continue to
step until the three-phase line expands. We accept the largest possible
contact angle as the advancing contact angle. Then we use the Step In
and Measure command to remove volume and measure until we reach the
smallest possible value which represents the receding contact angle.
This method works well for the advancing contact angle and typically is very close to the value that is achieved using the tilting base method. In the case of the receding contact angle, however, the presence of the needle does disturb the drop and makes it difficult to capture the true receding contact angle. The tilting base method offers a more reliable method -- at least as far as the receding contact angle is concerned.