Dry adhesion is very attractive to users as it does not leave behind any residues as compared to commonly used wet adhesive. This differs from suction which uses pressure differences and requires a smooth surface for its application. Gecko is able to scale up walls using dry adhesion from its feet. Careful examination of its feet structure reveals numerous hairs in the nanometer scale known as setae. To mimic the structure, electrospinning may be used to produce nanofibers. Baji et al (2013) have measured the adhesive pull off force of a single electrospun fiber to be about 18 nN which is comparable to natural fibers.
Adhesion from electrospun fibers may be between two electrospun surfaces or between an electrospun surface and another surface. Ballarin et al (2013) investigated the adhesive force between two electrospun surfaces based on fiber orientation using a peel test. Their studies showed that aligned electrospun polycaprolactone (PCL) fibers exhibits greater adhesion of about 760 kPa compared to randomly oriented fiber surfaces of about 610 kPa. It is suggested that the adhesion strength comes from asperities on the fibers and their geometric arrangement. With alignment, more fibers come into contact with one another from the touching surfaces compared to random fibers. This increases van der Waals adhesion forces between the touching surfaces.
In most applications of electrospun fiber based dry adhesive, it would be between fibrous surface and another surface. This is similar to gecko walking up glass panels or smooth wall. Using aligned nylon-6 electrospun fibers, Najem et al (2014) found that it has a maximal Mode II shear adhesion strength of 27 N/cm
2 on a glass slide which is 270% that of gecko feet. This Mode II shear adhesion strength is 97-fold greater than the Mode I (normal) adhesion strength. Where high shear adhesion strength is required and easy lifting is desirable, such a coating will be in demand.
Diameter of the fibers may have a significant effect on the shear adhesion strength. Najem et al (2014) showed that 50 mm diameter fibers exhibit substantially greater shear strength than 300 mm diameter fibers. With smaller fiber diameter, the packing density increases and this increases the surface contact between the electrospun aligned fibers with the glass surface and allows van de Waal forces to act. Using a shaft-loaded blister test instead of peel test also showed smaller diameter fibers exhibit greater adhesion than larger fibers [Chen 2013]. The study by Chen (2013) also showed increasing substrate surface roughness increases electrospun membrane adhesion onto it. With more surface asperities, greater mechanical inter-locking between the fibers and the substrate surface can be expected and hence greater adhesion. Comparing surface adhesion between two randomly oriented electrospun fiber surfaces and an electrospun surface adhesion on a cast film (smooth surface), the former resulted in 30% more adhesion energy due to asperities effect.
Apart from the diameter of the fibers affecting adhesive strength, the thickness of the membrane also has an influence. In general, shear adhesion strength has an inverse relationship with membrane thickness. Sahay et al (2017) found that the shear adhesion strength of electrospun poly(vinylidene fluoride) fibers (PVDF) of 200 µn; m thick was approximately 0.165 N/cm. In their study, the normal pull off strength was relatively high and constant over 1000 consecutive attachment-detachment cycles. This is in contrast with the observation by Najem (2014) where the normal pull off force is weak. The fibers in the work by Najem et al (2014) were aligned but those by Sahay et al (2017) were random and this may account for the difference in the adhesion characteristics.
Investigation of electrospun nanofibers and its dry adhesion properties is still at an early stage. There are still many questions that have yet to be answered such as whether the adhesion is dependent on material selection and if so, what are the characteristics of the material that favours better adhesion. Is there a universal membrane thickness for optimal adhesion? How would impurities on contact surface affect dry adhesion properties? More research and understanding in dry adhesion properties of nanofibers and further optimization of structure and perhaps material selection to increase adhesion strength of nanofibers will open up exciting new applications.
Published date: 07 March 2017
Last updated: -
▼ Reference
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Baji A, Zhou L, Mai Y W, Yang, Yao H. On the Adhesion performance of a single electrospun fiber. Appl. Phys. A (2015) 118: 51
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Ballarin F M, Blackledge T A, Davis N L C, Frontini P M, Abraham G A, Wong S C. Effect of Topology on the Adhesive Forces Between Electrospun Polymer Fibers Using a T-peel Test. Polym Eng Sci 2013; 53: 2219.
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Chen P. A Preliminary Discourse on Adhesion of
Nanofibers Derived from Electrospun Polymers. PhD Thesis 2013. The University of Akron.
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Najem J F, Wong S C, Ji G. Shear Adhesion Strength of Aligned Electrospun Nanofibers. Langmuir 2014; 30: 10410.
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Sahay R, Baji A, Ranganath A S, Ganesh V A. Durable adhesives based on electrospun poly(vinylidene fluoride) fibers. Inc. J. Appl. Polym. Sci. 2017; 134: 44393.
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