3. The future: Smart materials that can be dynamically tuned
Now that we have a good understanding of how important a role friction plays at the nanoscale and why it is no trivial task to deal with it, we can explore a variety of possible technology scenarios that researchers are contemplating.
One of the promises of nanotechnology is the ability to dynamically "tune" the surfaces of materials in order to improve their effectiveness. These functional materials are called "smart" because they can autonomously respond to changes in their environment reacting to stimuli such as temperature, light, pressure or electricity. This response is reversible and can be repeated.
Examples of smart materials are:
• Piezoelectric materials convert electrical energy to mechanical energy, and vice versa, and are used as actuators or energy harvesters.
• Shape memory alloys have the ability to change phase as a function of temperature, and in that process generate a force or motion. Examples are bendable eyeglass frames, fire protection systems or implant materials.
• Hydrogels are water-swollen structures that can be tailored chemically to respond to different stimuli. Their principal application area is in the biomedical and pharmaceutical industries, for instance for drug delivery.
A smart material that can be tuned in response to its environment possesses the ability to recognize certain environmental factors and then react, i.e. change its structure, in a predetermined way to respond to one or more of these factors. For our purpose in exploring friction, it would be the ability to modulate the friction of a surface.
For instance, many areas of robotics, particularly locomotion and grasping, can benefit from the ability to modulate friction of grippers and other contact surfaces. The results could be wall-climbing robots and novel gripping systems.
Another example is a medical device that might have varying degrees of how it makes contact with different kinds of biological substances. This would change the degree to which biological cells would stick to or get repelled by the device's surface. For implants, you would want bacterial cells to be repelled (to avoid infections) but bone and tissue cells to be attracted (to increase integration at the implant site).
Potential applications in biomedical systems are also driving the development of smart glues with controlled and tunable adhesion.
The conventional pressure-sensitive adhesives that you have at home are based on viscoelastic polymers, which are either strong & difficult to remove (like duct tape) or weak & easy to remove (like sticky notes).
A lot of research has been done on ways to enhance stickiness of adhesive materials through chemical treatments and microfabrication of insect-inspired hairy structures on the adhesive layer. Ideally, the adhesive properties of these materials could be tuned, so that you have the option of great stickiness or easy release, depending on what you need in a given situation.
But there are other ways of structuring materials in order to affect their friction behavior. Henri discovers several options that researchers have developed. Without getting to deep into the science behind these techniques, let's just take an overview. You can always read the scientific articles referenced in the Science Notes at the end of this chapter if you want to explore more or get into greater detail.
3.1 Inspiration from the Japanese art of kirigami
One intriguing approach is based on kirigami-inspired structures (see: Creating highly tunable adhesives with Kirigami-inspired structures).
Kirigami is an ancient Japanese art of cutting and folding paper to obtain 3D shapes. By introducing kirigami-inspired cuts into adhesive films a new mechanism arises to spatially control and enhance adhesion strength, where counter-intuitively, scientists at Iowa State University of Science and Technology, have shown that a cut adhesive film can be 10 times more adhesive than an uncut film.
By carefully designing arrays of cuts in an adhesive film, the stickiness can not only be tuned by a factor of ∼100 across a single sheet, but also be decreased for an easy-release purpose. This could lead to a variety of patterns to further enhance adhesion and easy-release characteristics of smart adhesive materials.
3.2 Nanofiber surfaces that can be tuned with light
The absorption of light provides some molecules with the energy necessary to switch from one chemical arrangement – what chemists call isomer – to another. The electronic and geometric differences between the isomers are responsible for changes in their physical properties, such as electrical conductivity, refractive index, or volume. This mechanism, called photochromism, could be exploited to make materials that are light sensitive.
To achieve friction tunability, responsive materials such as photochromic molecules could be very attractive systems since they change their properties reversibly under the action of an external stimulus.
Photochromic molecules can be controlled in such a way that they toggle back and forth between two different states, similar to an electronic device being switched to the on-position and off-position. This control is done with light. The wavelength of the light hitting the molecule determines which of two possible structures the molecule assumes.
Scientists in Italy have found that the frictional forces of nanofibrillar surfaces made of crystals from photochromic molecules gradually increased when exposed to UV light for short irradiation times and then returned to the initial value upon dark storage for 24 hours. These findings (see Langmuir, "Tunable Friction Behavior of Photochromic Fibrillar Surfaces") provide the possibility to develop a system that controllably and accurately generates both low and high friction forces that can be dynamically tuned.
3.3 Dry adhesives that can be switched on and off by magnetic fields
Researchers at Simon Fraser University in Canada have fabricated a magnetically controlled dry adhesive device by dispersing magnetic iron oxide particles in a particular type of silicon-based organic polymer (PDMS). The normal adhesion force in this device can be increased or decreased depending on the presence of an applied magnetic field (see: Dry adhesives controlled by a magnetic field).
In the presence of an applied magnetic field, the stiffness of the device's backing layer increases. The increase in the stiffness of the backing results in increased adhesion because a stiffer backing layer resists peeling. The adhesion force also depends on the orientation of the magnetic field.
3.4 Stimulus-responsive hydrogels
Stimuli-responsive hydrogels are a class of materials closely resembling biological tissues in their physical and chemical properties. These swollen polymer networks are of interest in research and industry for many biomedical applications due to their unique capability of a reversible volume change in response to different chemical or physical stimuli (temperature, pH, ionic strength, etc.). They are already used in tissue engineering, drug and cell delivery and wound healing.
A team of scientists from Duke University in the U.S. has found that certain hydrogels in a collapsed conformation exhibit significantly more friction than swollen gels (see Langmuir, "Switchable Friction of Stimulus-Responsive Hydrogels"). These differences arise from changes in the surface roughness, adhesive interactions, and chain entanglements of the gel surfaces associated with the phase transition from swollen to collapsed and vice versa.
Importantly, this research shows that the changes in friction, triggered by an external stimulus such as a change in temperature or solvent composition, are reversible. These reversible and possibly tunable changes in friction may have a significant impact on the design of coatings for biosensors and for actuation devices.
3.5 Reversibly tuning friction with polymer brushes
One possible tunable lubrication platform is based on counterion-driven interaction with polyelectrolyte brushes.
Let's first look at what counterions are. Imagine a sodium atom comes together with a chlorine atom to form table salt (sodium chloride with the chemical formula NaCL): The sodium atom lets one of its electrons go, and the chlorine atom snatches it up. Now there is a sodium cation and a fluoride anion. The two ions are held together by the attractive forces between their opposite charges. They have formed a salt. Here the sodium cation is the counterion for the chlorine anion and vice versa.
In one study (Macromolecules, "Dramatically Tuning Friction Using Responsive Polyelectrolyte Brushes"), scientists in China have explored how to dramatically tune surface friction by taking advantage of charged polymer in response to a richness of electrolytes. They demonstrated a tunable lubrication platform where the macroscale friction can be dramatically tuned from superior lubrication to ultrahigh friction progressively. This tunable friction is based on counterions' driven interaction with polyelectrolyte brushes.
3.6 Switching friction with electric fields
Researchers in the U.S. have investigated the possibility of using external electric fields as a means to actively control the friction response of a low-density self-assembled monolayer (SAM) film via atomic force microscopy experiments (Langmuir, "Nanoscale Friction Switches: Friction Modulation of Monomolecular Assemblies Using External Electric Fields"). SAMs are organic molecules, which form monomolecular layers – that means layers that are only one molecule thick – spontaneously when a solid substrate is immersed into a solution containing the molecules.
They found that there is a difference in friction response depending on the polarity of the electric field. They attribute this to the changes in the structural and crystalline order of the film caused by the electric field.
This kind of a ìfriction switchî could potentially be employed in micro- and nanoscale devices to control fluid flow (the relevant fields are called microfluidics and nanofluidics) or sliding of movable components (in MEMS and NEMS).