Lesson #2: Surfaces are the other player

Note: the following lesson is written in a manner to help provide understanding to a general user. This is neither a scientific nor technical document. More scientifically detailed understanding can be found in the following papers:                                          

Looking beyond fibrillar features to scale gecko‐like adhesion

MD Bartlett, AB Croll, DR King, BM Paret, DJ Irschick, AJ Crosby

Advanced Materials 24 (8), 1078-1083

High capacity, easy release adhesives from renewable materials

MD Bartlett, AJ Crosby

Advanced materials 26 (21), 3405-3409

Designing Bio‐Inspired Adhesives for Shear Loading: From Simple Structures to Complex Patterns

MD Bartlett, AB Croll, AJ Crosby

Advanced Functional Materials 22 (23), 4985-4992

Scaling normal adhesion force capacity with a generalized parameter

MD Bartlett, AJ Crosby

Langmuir 29 (35), 11022-11027

Lesson #2: Surfaces are the other player (download here)

Summary

• Until now, as consumers, our attention to different surfaces was largely driven by learning which ones a given tape will stick to

• We have learned to try to minimize damage but we put up with it

• If we want a device to grip to and peel from the wall, the surface plays a more important role

• With additional requirements of no damage, repeated use and holding weight, the surface’s role increases

• We now have to think about the types of surfaces and their role when considering reusable, non-damaging, load-bearing, multi-surface devices

Introduction

Surfaces play a FAR more important role in surface-mounting adhesion than the general user appreciates. That prominent role is not surprising if you think about it. The surface IS afterall one of the two involved parties. I surmise the reasons we haven’t given surfaces a lot of thought are five-fold. First, we’ve learned what tapes will work on which surfaces so that by now we have a sense of their performance. Second, we’re willing to put up with damage, however undesirable. Third, we’re asking that piece of tape to stick only once - it’ll either stick and/or damage or it won’t. Fourth, the adhesives from which we expect repeated use on any surface are not load-bearing, surface-mounting products.

The tape companies try REALLY hard to tell us how to use their products best (on which surfaces). The hard truth is that we users are complicit in tape damage (for example, users don’t read the instructions more often than not). On one hand we want tape company products to stick but THEN we don’t want them to damage (or leave residue). That’s a tall order!

Felsuma Lesson

Over the last seven years, I realized we consumers, over decades, have learned a great deal about adhesion to surfaces. But when it comes to load-bearing adhesives AND ones that we want to grip and peel repeatedly without damage on multiple surfaces, we have to introduce an active (previously passive) player in the desired event: the surface. Felsuma has received hundreds of calls asking, for example, to hold tens of pounds on painted drywall. Not so fast. As SOON as I tell those callers that the paint is the limiting factor, without fail, those users say, “I didn’t think about that!” An executive at another fastening company once said to me, “once you go beyond 5 lbs. on painted surfaces, you’re pulling off paint”. The essence of that statement is simply that we don’t know, for any given square inch of painted surface, how well that paint is adhering to the surface below it. The paint could be adhering tightly in one spot and then we move over three inches and the paint and wall have a completely different (not so tight) relationship. No adhesive can know about the paint’s adhesion to the wall below it, at any given point.

Now add the additional challenge of wanting repeated Grip & Peel™ capability. THEN add the additional challenge of making that device load-bearing. As I mentioned above, we’re willing to put up with damage from tapes because we realize some tapes will hurt certain surfaces. If we’re now gripping and peeling multiple surfaces repeatedly and holding weight, the expectation is that we’re not damaging. This is a very difficult problem to solve. THAT’s why ALL of the load-bearing grip and peel products out there today are tailored as best they can be for a positive experience for specific surfaces. What we have observed is they either 1) can only be used once on painted drywall or 2) can only grip and peel smooth surfaces such as glass and metal.

Why IS that?

In this lesson we’ll explore the many surface parameters that have bearing on a Geckskin device’s performance.

Let’s start.

Many, many people write to and call Felsuma asking for a device to grip a surface and hold a load. In making this request, most people fail to understand that it’s not the “sticking” part that is difficult - there are many, many very strong tapes on the market that can hold large amounts of weight - but try to peel them off and reuse them and they are not capable of anything but the sticking part. That’s what they were designed to do. The difficult part is the peel; that’s when the damage takes place. So, for the remainder of this document, we’ll talk about gripping to (with the intention of peeling from) a surface, not sticking to it.

For the most part, people divide surfaces into smooth and rough (or textured). Having now talked with multiple users and (adhesive) companies, I’ve learned that these two adjectives are not applied consistently. Furthermore, what some adhesive companies consider to be challenging surfaces others do not. In short, I’ve learned these two words, smooth and rough, are of limited value so we’ll use them carefully and define them specifically.

In Outline 1 below, let’s consider the following surfaces, in order from easiest to most difficult to grip and peel.

Outline 1

Surfaces:

• Easiest

  • Glass

  • Metal

  • Tile

  • Plastic^^

• Difficult

  • Finished and painted wood

  • Painted drywall

  • Painted cinder block

• Extremely difficult

  • Unpainted drywall

  • Unfinished wood

  • Unpainted cinder block

• Not possible with current devices

  • Skin

  • Hair

  • Fabric  

^^ Some plastics are actually quite difficult to stick or grip to. But there’s so much variability in plastics and there are many that one can grip and stick that I decided to put it in the “easiest” section but qualify that plastics span a broad range of materials and therefore surfaces

Before we dive into the above list and a surface by surface discussion, let’s establish some high level concepts.

Outline 2

Says something about the surface

• Dirt

• Surface pattern and design (e.g. porosity, gaps such as grout in tile)

• Water

Says something about the device

• Load bearing (or not)

• Reusability

• Damage and residue

• Multiple different surfaces

Just looking at the above two bulleted lists (excluding the “Not possible with current devices” list), we’re talking about a figurative 11x7 matrix of scenarios, variables, performance parameters, devices, etc. THAT’s how complex this topic is. We’re not going to address every cell in that figurative matrix. Rather this lesson will endeavor to provide foundational information so that you can extrapolate to fill in the figurative cells as you think about each one. This is what we call Think Like Geckskin. A Geckskin device does not stick, it grips, like a gecko. Therefore, it has a different relationship with the surface from say traditional tape; we’re asking for a different performance from a Geckskin device so the role of the surface is different as well. Think Like Geckskin is thinking about adhesion differently from how we’ve historically thought about it for tapes and other adhesive devices and fasteners. And the reason is, we now have to consider how we’re going to grip and peel repeatedly AND not damage AND bear a load AND on multiple different surfaces. This requires thinking in a new way.

In Outline 1 above, we have four surface groups: 1) easiest; 2) difficult; 3) extremely difficult; 4) not possible with current devices. In Outline 2 we have two high level concepts: 1) says something about the surface; 2) says something about the device. I’ve introduced Outline 2 because by talking in general terms about devices and surfaces you’ll better understand how to think about Outline 1. Outline 2 will start you thinking differently about surfaces; it’ll give you some boundaries to understand what makes gripping and peeling (and load-bearing on multiple surfaces without damage) a challenge.

Says something about the device

Let’s first address the second group in Outline 2 above called “says something about the device”. By starting with devices, we quickly learn how device performance teases out categories of surface, first smooth and textured and then more categories show themselves. Below you’ll find a rather complex Diagram 1 that highlights the top four performance parameters, to get us started. This is a big picture sort of (busy) diagram. I suggest you start from the left on the X axis and work your way right.

Diagram 1: Adhesive Device Performance from easier (more common) to difficult

Diagram 1 uses four performance parameters to exemplify the spectrum of difficulty (Y axis) v. performance (X axis) for surface-mounting adhesives. The notion of load-bearing** capacity creates performance divide in the world of adhesives. The second variable this figure highlights is repeated use. The third is smooth versus textured surfaces. And the fourth is damage or residue.

** Let’s spend a minute defining “load-bearing” and “load capacity”. These terms, in our Geckskin lessons, refer to “wall mounting products” designed specifically to hold something on a wall. A hook that one screws into a wall, for example, is designed to hold something on the wall. I’m making this distinction because a strong strip of say, masking or duct tape, can hold weight, to be sure. But that’s just one of the many uses of that tape. Therefore, load-bearing and load capacity are referring to the capability of so-designed wall mounting products to hold an object on the wall.

Clearly Diagram 1 is a simplification of the real world, but it does begin to tease apart large categories of surface-mounting products’ performance. By starting with device performance we begin to understand the role of the surface to achieve certain performance requirements.

What we can conclude about the general, incumbent wall-mounting products on the market (excluding Geckskin devices)  from the (ridiculous) Diagram 1:

●     There are tape products that can stick once and damage or leave residue (by the way, many are quite strong and can support substantial weight)

●     There are products that can stick repeatedly BUT 1) only on smooth surfaces if they’re load-bearing or 2) they are non load-bearing if they perform on any surface

●     There’s one load-bearing product that can stick to any surface once

Another way of looking at the above, there are products in the market today if we are willing to eliminate one of the four performance parameters. However, since our objective in this lesson is to learn about how to grip and peel 1) repeatedly, 2) without damage, 3) on any surface, 4) while bearing a load, we’ll now turn our attention back to the role of surfaces in that pursuit.

Says something about the surface

Now turn to the first group in Outline 2 called “says something about the surface”. These parameters will help you understand the surfaces better as you realize what makes gripping and peeling on one surface easier than another. In short, dirt, surface pattern and design, and water are all impediments to establishing a sound grip; a Geckskin device requires surface area to establish the grip. For those of you who are familiar with it, this is where Van der Waals forces## come into play. We require sufficient surface area for Van der Waals forces to enable reversible load-bearing. Our device cannot grip if the device has insufficient surface area due to size of the Pad (that’s for another lesson) or from surface disruptions that decrease the intimate contact of our Pad to the substrate. Anything that can impede that grip will make it difficult for a Geckskin device to perform. Dirt and water come between the device and the surface. Surface pattern and design includes such instances as porosity or interruptions in a surface such as grout between tiles. Porosity and grout, for example, reduce surface area, making it difficult to establish a grip. Now let’s combine the question of water and surface design. We receive many questions about whether a Geckskin device can work with water. Table 1 below depicts three different scenarios for how a Griphook, for example, will perform with water and surface design as variables.

## To understand Van der Waals forces, here’s one reference (there are many on the web): https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/van-der-waals-force

Table 1. Water and Surface Design performance limitations

* Solid metal, tile, glass

** straddling two tiles, porous tile, painted dry wall, wood, etc.

Dirt is a challenge. How and when a device will get dirty will have a direct impact on its performance. People call to ask about Geckskin devices: “how many times will it work?” The answer is, I can’t answer that. If you drop it in the dirt upon opening it, it may not work once. If you use it on a dirty surface, that’ll reduce the number of times you can use it. Furthermore, different Geckskin devices will react differently in the presence of dirt (remember, Geckskin is a technology not a material). Ours are still adhesive products so they will get dirty and the dirt will make it difficult to grip surfaces and their performance will deteriorate. But how fast that performance deteriorates will be a function of the type and quantity of dirt AND the surface you wish to grip (i.e. it’s easier to grip glass than painted drywall).

By starting with the above two groups addressing surface impediments and device performance, hopefully you can now understand better why we have categorized the surfaces as we have from easy to extremely difficult.

Using what we’ve just learned, let’s turn our attention to the first set of bullets in Outline 1 where we group the surfaces from easy to extremely difficult, I’ll quickly comment on each group.

Starting with the easy surfaces first, metal and glass, these two truly are smooth surfaces. Smooth surfaces are the easiest to grip and peel because those surfaces offer the maximum surface area for a mounting device to grip. Many load-bearing devices can grip and peel metal and glass repeatedly. The other reason they are optimal is it’s difficult to damage them being unpainted surfaces.

The issues with tile are two-fold. One is the gap between tiles (grout) and the second is porous tile. Both of those surface designs impede the ability to create a bond. Plastics may seem like an easy surface but there are many types of the plastics and the chemistry of different plastics can be problematic - polypropylenes are not very inviting to gripping adhesives due to their low surface tension. If the manufacturer has sprayed on a mold release coating, that can impede some plastics from gripping as the plastics will “bloom”, meaning chemicals will migrate to the  surface over time. That release of an ingredient can create a peel opportunity (loss of Peel Control™ capability) over time. Summarizing, some of the gripping challenges with plastics are:

●     Chemistry/surface tension of the plastic

●     Spray on mold release

●     Color concentrate or other additive can migrate out

The next category is difficult surfaces. They’re difficult primarily because of porosity and texture but also because they have a layer of painted surface. Wood, drywall and cinder block are textured surfaces. The addition of paint or finish (in the case of wood) to those surfaces brings both a benefit and challenge. The benefit is that the paint or finish makes for a smoother surface and therefore easier to establish a grip - more surface area, less porosity and dust. The challenge is we have two new players in the load-bearing equation - the paint and the “paper” veneer on the compressed material (or whatever the board is made from)! The paint and the paper become the limiting factors. We don’t know the adhesive strength of those materials to each other. That board-to-veneer-to-paint adhesion can change with different paints, different walls and every few inches!! If the paint and/or the veneer has little adhesive strength to the surface below, painted drywall won’t hold much weight at all; the device’s capabilities become irrelevant.

Next, let’s discuss the extremely difficult surfaces. Hopefully by this point you can understand what makes them so difficult: porosity and dust. Unpainted surfaces are (in addition to being highly textured) porous and covered with a layer of dust. Gripping and peeling such surfaces is indeed, extremely difficult. The porosity reduces the surface area and then the layer of dust comes between the Geckskin device and the surface. Please remember, the goal is to achieve all four performance parameters. If you want to stick once, there are many (strong) tapes that will stick to these surfaces. But when it comes to load-bearing wall-mounting products, there are none, except one, BUT you’ll have to relax one parameter: repeated use. Felsuma has a device at Felsuma called the Griphook FM™ product. It will grip unpainted wood once, while holding weight. You may see a video of the Griphook FM in action here: https://youtu.be/iKmMCx5IG_g

If we’re willing to relax one other parameter, which is load bearing, then Felsuma has other products that will grip these extremely difficult surfaces. The Griptile™ product will sometimes grip these (depending on just how porous or dusty the surface is) but the Griptile will hold no weight. So there are tradeoffs such as load-bearing or number of uses as the surfaces become more challenging.

Lastly, we have the list of surfaces that are not possible with current devices. Skin may seem like a dense surface but there are hairs, sweat, pores and a lot of dynamic motion. The Sci Fos (Geckskin scientific founders - the two UMass Professors who invented this technology) are working on a Geckskin device to grip skin. If you’d like to learn more about that, please contact us and we’ll be happy to introduce you to Professors Crosby and Irschick. Hair is not a surface in the way we’ve been discussing it. Fabrics are porous, dynamic (like skin) and may have threads. Those threads, similar to dust on say brick, are not a surface we can grip.

Here endeth the lesson.

The objective of this lesson was to bring to light the active role the surface plays if we want to be able to take advantage of all four performance parameters in a Geckskin device: load-bearing, repeated use on multiple surfaces and no damage. Hopefully this lesson has conveyed the importance of the role the surface plays when thinking about using a wall-mounting product with all four performance parameters. Take note, IF we’re willing to relax one of the four parameters, we can add more devices (both Geckskin and non-Geckskin) that can perform with the remaining three.