Mechanical Engineering Faculty Research

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To bio-mimic gecko’s foot hair, which possess high adhesion strength and can be re- usable for lifetime, fibrous membranes are fabricated by electrospinning to provide sufficient adhesion energy. Shaft-loaded blister test (SLBT) is firstly used to measure the work of adhesion between electrospun membrane and rigid substrate. Poly(vinylidene fluoride) (PVDF) were electrospun with an average fiber diameter of 333±59 nm. Commercial cardboard with inorganic coating was used to provide a model substrate for adhesion tests. In SLBT, the elastic response PVDF was analyzed and its adhesion energy measured. FEA model with cohesive layer is developed to evaluate the experiment results. The results show SLBT presented a viable methodology for evaluating the adhesion energy of electrospun polymer fabrics. Electrospun membranes with different fiber diameter are tested for their distinctive adhesion property. Five sets of PVDF membranes with different fiber diameters (from 201± 86 nm to 2724± 587 nm) are electrospun for size effect evaluation. Obtaining testing results from SLBT adhesion test, adhesion energy ranges from 258.83±43.54 mJ/m2 to 8.06±0.71 mJ/m2 . Significant size effect is observed, and electrospun membrane composing from finer fibers possesses greater adhesion energy. Thickness effect is also evaluated. By stacking multiple layers of electrospun membrane together, iv membrane samples with different thickness are produced. Test results illustrate thick membrane trends to debond easier than thin membrane. After considering the characteristic of electrospun membrane, the effect of substrate is also evaluated. One approach is made by substituting SiC substrates with different roughness for cardboard substrate. The grit size of the SiC substrates varies from 5 µm to 68 µm. A correlation between adhesion energy and mean peak and valley roughness (Rz) is established from mechanical interlocking theory. The other approach is comparing adhesion energies if substrate is cast film or elctrospun fibrous substrate. Between electrospun PCL membranes, adhesion energy is exhibited at 305.0 ± 41.9 mJ/m2. This value is 1.32 times larger than the adhesion energy between electrospun PCL membrane and cast PCL film. The high adhesion energy is attributed to the large surface contact and interlocking effect initiated by the amorphous fiber morphology of the electrospun PCL membranes. The results establish a novel methodology and provide a feasible way to control the adhesion properties of electrospun membranes. In the end, a unique approach to fabricate PVDF/PVA hollow fiber structure is presented. Hollow structure is potential used for mimicking muscular contraction and extension, which will need to fill with functional fluid into the fiber. The fabrication methodology includes co-axial electrospinning of PVDF and PVA solutions, and a water assisted route to mitigate secondary erosion. Without solvent erosion, PVDF/PVA fibers exhibit smooth inner and outer surfaces and hollow structure. Furthermore, the hollow fiber diameter and wall thickness are controllable by the feed rate of PVA solution in v electrospinning. SLBT test is adopted to determine the adhesion energy between hollow fiber and rigid substrate. The hollow structure exhibited better adhesion performance compare to solid fiber in similar diameter. Overall, SLBT test is adopted to determine the adhesion energy of electrospun membrane for the first time. Size of the fibers, thickness of the membrane, topography of the substrate, loading speed and materials of the substrate are the considered parameters in this study. Contribution is made to establish adhesion mechanism of electrospun membrane. Applications of the electrospun membrane are developed for potential nanoconnector and hollow piezoelectric fibers.



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