Sent May 17, 2002

A proper understanding of the terms adhesion and sticktion is critical to one's effectiveness as a pedestrian slip resistance practitioner. Lately a number of callers have attempted to discuss the validity of various slipmeters on wet surfaces without understanding what separates the men from the boys in wet testing. When I recently went back to my Pedestrian Slip Resistance book [http://www.englishxl.com/cont.htm] and my Web page on Learning to Speak Technobabble to check out what I had said in print, I saw that I had not fully developed the explanations. I therefore modified the text of http://www.englishxl.com/techbab.html to fill in the blanks a bit.

Slip resistance is defined in ASTM F1637.95, Practice for Safe Walking Surfaces as The relative force which resists the tendency of the shoe or foot to slide along the walkway surface. Slip resistance is related to a combination of factors including the walkway surface, the footwear bottom, and the presence of foreign material between them. There are almost as many spellings of the sticktion term as there are investigators writing about it, though none of them will show up on your word processor's spell checker. When attempting to measure the slip resistance of a walking surface, it is more important to avoid it than it is to spell it right, however.

Adhesion, or sticktion, arises as a function of the slider pad's residence time on the surface. If there is any delay between the instant of surface contact and the application of the horizontal movement, sticktion or adhesion will occur. Water is a very tenuous film, and any residence time of the slider shoe on the wet surface causes the water film to be squeezed out of the interface between the shoe bottom and the walking surface so that the tendency to slip is greatly reduced. This is called sticktion. It is the squeezing out of the water film between the shoe bottom and the floor due to residence time.

On dry surfaces, the longer the residence time, the greater adhesion will be. On surfaces wet with water, residence times shorter than .2 second (that's 200 ms in PhD-speak) are known to produce significant sticktion that can result in slipmeter readings that may be higher under wet conditions than would be obtained on the same surface in a dry state. All dragsled meters and all articulated strut instruments that do not apply the horizontal and normal forces simultaneously with initial surface contact are afflicted with disqualifying sticktion. That is, you can't take a valid reading on a wet surface with any dragsled instrument, regardless of what the testing standards say, even if your dragsled weighs 50 pounds!

Certain investigators have remarked about what they call a "suction cup effect" that sometimes occurs under shoe bottoms or slipmeters on wet surfaces, but this is not sticktion as defined herein. There is no evidence that the suction effect increases slip resistance. In fact, it appears to promote greater slipperiness under some tennis shoes as water trapped in the cavity created by "suction cups" is forced out under hydraulic pressure as weight is increasingly applied, thereby breaking the sticktion that had been initially achieved. Some investigators have compared sticktion to what you get when you put two wet microscope slides together. They do tend to stick together and resist pulling apart, but the water does not inhibit horizontal relative movement between them. They slide freely on the water film. This is not sticktion. Sticktion is what you get with your XL slipmeter when you actuate it on a wet surface while holding the cylinder back against the rubber stop so that the shoe doesn't slide. After a brief moment of pressure on the shoe in contact with a wet surface, its tendency to slide is greatly reduced.

For example, if you meter a sheet of glass, you might get .12 wet. By holding back the cylinder for a moment before releasing it, you can get numbers that are much higher. This is sticktion, and it is an illustration of why no meter with a residence time can possibly be used for metering wet surfaces. It is what separates the XL from the dragsleds and articulated strut machines.

It is possible to get a suction effect with large slider pads on smooth surfaces like a glazed ceramic tile that will actually lift the tile up off of the surface. That may occur simultaneously with sticktion, but it is not sticktion. It is important to use these technical terms correctly if you are to be persuasive as a pedestrian tribologist.

Somewhat related is the difference between coefficient of friction and slip resistance, which is adequately discussed on http://www.englishxl.com/techbab.html.

What is the Threshold of Safety?
A number of callers don't understand that the traction required for safe walking is a function of the ground reaction forces applied by the pedestrian's shoes to the walking surface. It has nothing to do with whether the surface is wet, dry or greasy. The widely-accepted .50 as an unquestionably safe available traction number for ordinary walking applies no matter what the pedestrian is walking on. [See http://www.englishxl.com/point5.html for the most complete discussion you will find on this topic.]

Some slipmeter users seem surprised that the .50 threshold applies to contaminated surfaces as well as dry. This can only be because they don't understand that clean, dry surfaces are not slippery under most rubber shoe bottoms, and it is the surface contaminant that generates the hazard in the vast majority of slip incidents. That's why the XL was designed for testing wet or otherwise lubricated surfaces primarily.

What about Mats?
Several callers have asked about metering traction on floor mats. I have referred them to Chapter 3 in my Slips, Trips and Falls book and ANSI A1264.2 for guidelines on mats and runners. Often the problems introduced by the use of entry or walk-off mats are almost as great as the problems they were designed to remedy, especially if the wrong type of mat is used and it is not properly maintained. Most mat-related falls are trips arising from wrinkles in the mat on an exposed edge that is high enough and sharp enough to trip the walker. Mats are seldom slippery, except when the wrong mat is used for the contaminant present, which can attack the mat chemically so as to change its traction properties. Mats should be large enough to wipe the pedestrian's shoes several times while traversing the mat, and entry mats should be made so that they don't get saturated and re-wet the shoes walking across them.