This study introduces a novel method for removal of particulate contamination – loosely referred to as dust – from solid surfaces using polymeric μ-dusters. In this study, we illustrated for the first time that polymeric microfibrils of controlled interfacial and geometrical properties can effectively remove all micrometric and submicrometric contaminant particles from a solid surface without damaging the underlying substrate. Once these microfibrils are brought into contact with a contaminated surface, due to their soft and flexible structure, they develop intimate contact with both the surface contaminants and the substrate. While these intrinsically non-sticky micropillars have minimal interfacial interactions with the substrate, we showed that they produce strong interfacial interactions with the contaminant particles, granting the detachment of the particles from the surface upon retraction of the cleaning material. Unlike flat substrates of the same material, using microfibrillar structures of controlled interfacial and geometrical properties also allows the elimination of the adsorbed particles from the contact interface. By moving the adsorbed particles from the tip to the side of the fibrils and consequently removing them from the contact interface, polymeric microfibrils can clean all contaminant particles from the surface.
Izadi, H.; Dogra, N.; Perreault, F.; Schwarz, C.; Simon, S.; Vanderlick, K. T.; “Removal of Particulate Contamination from Solid Surfaces Using Polymeric Micropillars”, ACS Applied Materials & Interfaces, 2016, 8, 16967-16978. DOI: 10.1021/acsami.5b09154
Izadi, H.; Vanderlick, T. K.; “Systems and Methods for Particulate Removal Using Polymeric Microfibrils”, U.S. Patent, October 2015
Gecko Adhesion & Contact Electrification
Geckos, who are capable of walking on walls and hanging from ceilings with the help of micro/nanoscale hierarchical fibrils (setae) on their toe pads, have become the main prototype in the design and fabrication of fibrillar dry adhesives. Since the unique fibrillar feature of the toe pads of geckos allows them to develop an intimate contact with the substrate the animal is walking on or clinging to, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification phenomenon. Even so, the possibility of the occurrence of contact electrification and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos has been overlooked for several decades. In this study, by measuring the magnitude of the electric charges, together with the adhesion forces, that gecko foot pads develop in contact with different materials, I have clarified for the first time that contact electrification does contribute effectively to gecko adhesion. More importantly, I have demonstrated that it is the contact electrification-driven electrostatic interactions which dictate the strength of gecko adhesion and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.
Izadi, H.; Zandieh, A.; Penlidis, A.; “Bio-inspired Dry Adhesives: Contact Electrification and Electrostatic Interactions”, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., 2017. DOI: 10.1002/0471238961.koe00034
Izadi, H.; Stewart, Katherine M. E.; Penlidis, A.; “Role of Contact Electrification and Electrostatic Interactions in Gecko Adhesion”, Journal of the Royal Society Interface, 2014, 11, 20140371. DOI: 10.1098/rsif.2014.0371
How Does a Gecko-inspired Adhesive Stick?
With respect to the dry adhesion mechanism that geckos employ for their locomotion, it is commonly accepted that adhesive performance of synthetic bio-inspired dry adhesives is coming from the formation of van der Waals interactions at the tip or side of the dry adhesives fibrils with the substrate they are brought into contact with. However, what has been usually neglected in this connection is that electrostatic interactions may also be developed at the contact between any two materials via the familiar contact electrification phenomenon. Although contact electrification is common and can have a large influence on interfacial interaction forces, its impact on adhesive properties of synthetic dry adhesives has been overlooked. Even so, recent reports on fabrication of polymeric dry adhesives, which can generate strong adhesion forces relying on electrostatic interactions coming from contact electrification, have brought to light again the idea that charging the surface of dry adhesives – specifically the polymeric ones – can play a very crucial role in their adhesive behaviour. From this perspective, the main reasons that have caused the lack of attention to this concept and the possible contributions of contact electrification in interfacial interactions of polymeric dry adhesives are thoroughly discussed in this critical review.
Izadi, H.; Penlidis, A.; “Polymeric Bio-inspired Dry Adhesives: Van der Waals or Electrostatic Interactions?”, Macromolecular Reaction Engineering, 2013, 7, 588-608. DOI: 10.1002/mren.201300146
Teflon AF Nanopillars
In this research, for the first time, a non-sticky fluoropolymer (Teflon AF) was employed to fabricate a novel dry adhesive consisting of high aspect-ratio nanopillars terminated with a fluffy top layer. Both the nanopillars and the terminating layer were fabricated concurrently by replica-molding using a nanoporous anodic aluminum oxide membrane as the mold. Concurrent fabrication of the terminating nanostructure helps the fabrication of extremely high aspect-ratio nanopillars which, up to an aspect-ratio of 185, neither collapse at the tip nor bundle. In order to fabricate nanopillars of different topographical properties, the height of the nanopillars as well as the size and density of the terminating nanostructure were carefully controlled by adjusting the processing temperature. In this research, a novel replica-molding technique for fabrication of bi-level Teflon AF nanopillars was also developed. Unlike conventional polymer infiltration methods which consist of filling the mold by only heating the polymer above its glass transition temperature, in this method, the polymer melt was also simultaneously cooled down during the infiltration process. Concurrent cooling of the Teflon AF melt allowed control over the interfacial instabilities of the polymer thin film, which formed ahead of the polymer melt upon its infiltration into the alumina nanochannels. Doing so, the geometrical properties of the peculiar fluffy nanostructure on top of the extremely high aspect-ratio Teflon AF nanopillars were modified. The electrostatically-driven adhesion of Teflon AF nanopillars helps them to generate extremely large adhesion and friction forces, up to ~2.1 and 1.2 times larger than those of a gecko toe pad, respectively. This type of Teflon AF nanostructures is also shown to have an exceptional capacity to generate strong adhesion forces under water, achieving even up to ~70% of the adhesion forces attainable in dry conditions.
Izadi, H.; Sarikhani, K.; Penlidis, A.; “Instabilities of Teflon AF Thin Films in Alumina Nanochannels and Adhesion of Bi-level Teflon AF Nanopillars”, Nanotechnology, 2013, 503306. DOI: 10.1088/0957-4484/24/50/505306
Izadi, H.; Golmakani, M.; Penlidis, A.; “Enhanced Adhesion and Friction by Electrostatic Interactions of Double-Level Teflon Nanopillars”, Soft Matter, 2013, 9, 1985-1996. DOI: 10.1039/C2SM27329B
Izadi, H.; Zhao, B.; Han, Y.; McManus, N.; Penlidis, A.; “Teflon Hierarchical Nanopillars with Dry-Wet Adhesive Properties”, Journal of Polymer Science Part B: Polymer Physics, 2012, 50, 846-851. DOI: 10.1002/polb.23076