In Situ Growth of Mushroom-Shaped Adhesive Structures on Flat/Curved Surfaces via Electrical Modulation

Abstract

Gecko-inspired adhesives have an extraordinary impact on robotic manipulation and locomotion. However, achieving excellent adhesive performance on curved surfaces, especially undevelopable surfaces, is still challenging. This can be attributed to a considerable difference between the fabrication method and practical necessity, i.e., the adhesive structures are generally fabricated on a flat substrate whereas the manipulating surface is curved, resulting in a low adhesive strength. Here, an in-situ growth strategy is proposed to fabricate mushroom-shaped structures at micro/nano-scale via electrical modulation on flat or curved surfaces. Since the adhesive structures are directly grown on target surfaces without a transfer procedure, they exhibit a large contact area and stress uniformity at the interface, corresponding to an excellent adhesive force. A comparison between grown structures using the proposed method and those fabricated using traditional approaches suggests that the adhesive forces are identical for flat testing surfaces, while the difference can be up to 4 times for developable surfaces and even 25 times for undevelopable surfaces. The proposed adhesion strategy extends the application prospects of gecko-inspired adhesives from flat surfaces to curved ones, composed of developable and undevelopable surfaces, opening a new avenue to develop gecko-inspired adhesive-based devices and systems.

Keywords: curved surface; dry adhesive; electrical modulation; in situ growth.

Figure 1

Schematic illustration of the in situ growth of mushroom‐shaped adhesive structures on numerous target surfaces. A,B) Schematic of mushroom‐shaped structures electrically grown on flat and curved surfaces, respectively. C) Mushroom‐shaped adhesive structures grown on flat surfaces with 100 × 100 mm areas. The scale bars in the SEM image and inset are 400 and 60 µm, respectively. D) Grown mushroom‐shaped adhesive structures on the concave surface with a 50.8 × 53 mm area. The scale bars are 400 and 100 µm, respectively. E) Adhesive structures grown on various substrates such as metal and PET. F) Adhesive structures grown on curved surfaces with different geometry, consisting of concave and spherical surfaces. G) Grasping ability of grown structures on different target surfaces, namely a weight as a flat surface of 5 kg, a convex plate as a concave surface of R = 30 mm, a beaker as a convex surface of R = 31 mm, and a globe holder as a spherical surface of R = 30 mm.

Figure 2

Analysis of the mushroom‐shaped adhesive structures grown under an electric field. A) Numerical simulations obtained the dynamic evolution of polymer film under an external electric field. Here, blue and red colors represent air and polymer, respectively. B) Electric field distribution at the air–polymer interfaces corresponding to different stages (i–iv) in A. C) Variation in electric field ΔE and polymer height versus evolution time. D) Evolution of mushroom tip diameter with the growing time and the change in contact angle on three‐phase boundaries. The macroscopic contact angle and the microscopic contact angle are marked in blue and green in the inset at 3.8 ms, respectively. E) Influence of applied voltage and air gap between the upper electrode and the polymer surface on the electric field ΔE. Grown adhesive structures on F) curved surfaces (R = 38.76 mm) with different voltages and G) varying gaps of air. The scale bars are 100 µm. H) Grown adhesive structures on convex surface (R = 10.34 mm), concave surface (R = 31.01 mm). and spherical surface (R = 13.11 mm). The scale bars are 400 µm. I) Grown adhesive structures on a convex cylinder with a curvature radius of 51.68, 38.76, 19.69, 15.5, and 10.34 mm, respectively.

Figure 3

Adhesion enhancement mechanism of the grown structures on curved surfaces. A) Dynamic behavior of the grown and sticking structures on curved surfaces at (I) initial state, (II) contact stage, (III) inversion state, and (IV) separation state; cloud atlas representing the internal stress. B) Evolution of the contact line for different adhesive structures as a function of process time. C) Evolution of the damage energy for different adhesive structures as a function of process time. D) Adhesive force as a function of process time for different adhesive structures. E,F) Stress at the interface between the adhesive and curved surface at contact and inversion states. G) Evolution of interfacial stress acting on the damage limit in the separating procedure.

Figure 4

Adhesion of the grown structures on diverse testing surfaces. A) Serial scenes are designed to test the adhesive performance, composed of flat surface versus adhesive structure, concave column versus adhesive structure, convex column versus adhesive structure, and concave sphere versus adhesive structure. B) Variation in the adhesion strength as a function of preload force for the flat testing surface. C) Influence of applied voltage on the adhesive performance of grown structures on flat surfaces. D) Variation in the adhesion strength on convex test surface as a function of preload force for tested samples, grown, sticking, and flat structures. E) Variation in the adhesion strength on concave test surfaces as a function of the preload force for the tested samples, composed of grown and stick structures. F,G) The adhesive performances of the grown and sticking structures on testing surfaces with various curvatures, respectively. H) Comparing the adhesive force of grown and sticking structures to several curvature radii. I) Variation in the adhesion strength as a function of preload force for the testing surface of an undevelopable spherical surface. J) Test cycles of adhesion performance of grown structure on the spherical surface.