In‐Plane Combination of Micropillars with Distinct Aspect Ratios to Resist Overload‐Induced Adhesion Failure

Abstract Bioinspired micropillar adhesives have shown broad application prospects in space capture and docking, due to their strong adhesion, good environmental adaptability, and reusability. However, when performing space missions, unavoidable contact collision with target objects may cause large deformation of the micropillars, resulting in the loss of adhesion ability. This study reports a novel micropillar adhesive through the in‐plane combination of micropillars (IPCM) with different aspect ratios, consisting of small pillars for retaining strong adhesion and large ones for resisting overload‐induced adhesion failure. It is demonstrated that the IPCM array can still maintain 85% of the adhesion peak after static large deformation compared to a general micropillar array composed of the same pillars. The impact of element size and layout of the IPCM, as well as detachment velocity on adhesion performance under high preload is discussed. Furthermore, finite element contact analysis qualitatively reproduces the experimentally observed micropillar deformations and attributes the overload‐induced adhesion failure to the redistribution of surface normal stress. Finally, the potential application of the IPCM in dynamic capture is demonstrated on different objects. The proposed IPCM opens up new design concepts for practical applications of bioinspired adhesives in space capture and docking.

In-plane combination of micropillars with distinct aspect ratios to resist overload-induced adhesion failure Dongwu Li 1 , Ruozhang Li 2 , Kangbo Yuan 3 , Ao Chen 2 , Ning Guo 1 , Chao Xu 1, * , Wenming Zhang 2, * 1 School of Astronautics, Northwestern Polytechnical University, Xi'an 710072, China                  Table S1.Characteristic parameters of all tested samples.The symbols h and λ denote the height and aspect ratio of pillars, respectively.The area proportion is defined as the proportion of the micropillar area in the total platform area, and the volume proportion is defined as the proportion of micropillar volume in the total space.

Figure S1 .
Figure S1.(a) Optical microscopic top view photo of a general micropillar array sample, (b) optical microscopic top view photo of an in-plane combined micropillar (IPCM) array sample, (c) partial side view of the general micropillar array in (a), each large micropillar has an aspect ratio of 3 (with a diameter of 100 μm), (d) partial side view of the IPCM array in (b), the aspect ratio of each large micropillar is 2 (with a diameter of 200 μm) and that of each small micropillar is 3 (with a diameter of 100 μm).

Figure S2 .
Figure S2.Photograph of customized test apparatus for adhesion measurement.The manual positioning state and the motorized translation stage are separately placed on an optical vibration isolation platform.A precise linear translation stage with closed-loop control is used to provide compressive and tensile loads.A camera is fixed above the test sample for capturing the contact images.The contact force and displacement on the adhesive surface are measured using a load cell and an built-in displacement sensor.

Figure S3 .
Figure S3.(a) Measured force-displacement relation of the IPCM array sample under a compression depth of 100 μm in which case no bending deformation of micropillars is induced during the entire loading and unloading process.(b) In situ pictures of the contact area between micropillars and rigid

Figure S4 .
Figure S4.Force as a function of time at different detachment velocities, 50 μm/s, 100 μm/s, 1000 μm/s and different compression depths (a) 40 μm, (b) 160 μm, (c) 320 μm, (d) 480 μm.(e) Pull-off force at different detachment velocities and different compression depths.It can be seen that the compression depth (or preload) does not affect the rate-dependent adhesion.

Figure S5 .
Figure S5.(a) Finite element model of a single micropillar meshed using 8-node linear brick element with a 2.5 MPa elastic modulus and a Poisson's ratio of 0.49, (b) bilinear cohesive zone model for simulating the adhesion interactions between micropillars and the probe.

Figure S6 .
Figure S6.Comparison of the experimental contact images of a micropillar against the contact contour obtained from finite element contact analysis under monotonically increasing compressive load.As the compressive load increases, the contact state gradually changes from 'full contact' (top contact) to 'no contact' (side contact).This qualitative comparison verifies the accuracy of the finite element analysis model.

Figure S7 .
Figure S7.Illustration of the displacement-controlled testing method including three stages, namely loading, pause, and unloading.During the loading and unloading stages, the displacement increases and decreases monotonically at a constant speed, respectively.The pause phase of all tests lasted 10 seconds.

Figure S8 .
Figure S8.Photograph of the test setup simulating dynamic capture.A pendulum rod can be manually controlled to collide with the bio-inspired adhesive at a certain initial speed.The speed of the rod and contact force are measured using a laser vibrometer and a load cell.

Figure S9 .
Figure S9.Three different target objects in the dynamic capture demonstration test: (a) a plano-convex lens with 83.3 mm diameter, (b) an aluminum alloy thin-walled tube with a diameter of 13 mm, (c) a piece of acrylic plate covered with polyimide film.