24th International Colloquium Tribology - January 2024 93 Mechanical Adhesion with Micropatterned Surfaces Translating Friction and Elastic Energy to Adhesive Forces Marco Bruno 1,2,3 , Luigi Portaluri 1,2 , Luciana Algieri 1 , Stanislav Gorb 4 , Massimo De Vittorio 1,2 , Michele Scaraggi *1,2,5 1 Center for Biomolecular Nanotechnologies, Italian Institute of Technology, Arnesano (LE), IT 2 Department of Engineering for Innovation, University of Salento, Lecce, IT 3 DIBRIS, University of Genova, Genova, IT 4 Zoological Institute, University of Kiel, Kiel, DE 5 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK * Corresponding author: michele.scaraggi@unisalento.it 1. Interlocking as a new tribological route to enhance and tune adhesion Controlling adhesion in harsh environmental conditions is of primary importance in many fields: From suture in surgical operations to wound healing tapes or underwater adhesives and grippers for robotics applications. [1] Surface interactions happen at all scales and almost at any time in any physical system and nature provides many examples of morphological features of animals, plants and micro-organisms (mosquitos, parasites proboscis or plant diaspore structures) [2] [3] [4] that can translate friction and elastic energy stored through interlocking mechanisms in adhesion forces, [5] any of these examples involves a compliant surface to store elastic energy. Since highly soft materials are becoming more and more common and easy to produce it is possible to use and tune their overall compliance by shaping their structure and changing their mechanical properties. This kind of mechanism is capable to operate in conditions that can compromise or completely negate the effect of standard adhesives: surfaces exposed to water, solvent vapors, lubricants [6], high temperatures, dusty environments, surfaces operating in high vacuum or aerospace applications. In this work we present various micro-patterned surfaces that can adhere to each other and once they are locked they can sustain high mechanical loads in normal direction. A simplified theoretical framework based on Hertz theory has been developed to predict the behavior of such systems. Through stereolithographic (SLA) 3D printers and soft lithography techniques we fabricated soft samples of the proposed geometries and tested them using a custom made opto-mechanical tribometer. Figure 1: (A) Soft-soft interlocking surfaces after printing and curing; (B) Detail of square-lattice patterned soft surface; (C) Schematic representation of the interlocking process indicating the main geometrical parameters of the structures. 2. Theoretical framework In order to build a model we will consider a single microstructure constituted by an elastic sphere and a rigid element that connects the sphere to the substrate as in Figure-1 (C) If we write the equilibrium equation in y direction and we couple it with the force vs penetration relation for Hertzian contact we obtain: Where F n is the normal force at the contact, F t is the friction force at the contact, F pull is the force in the pull-off direction, δ is the penetration, I is the distance between two neighboring structures, R is the radius of the spheres, E* is the effective Young modulus, y is the distance in the pull-off direction and m is the friction coefficient. We can then solve it for F pull as a function of y: We can then integrate this quantity with respect to y from first to last contact point to obtain the predicted (approximated) work of adhesion to be compared with the experiments, and multiply by the density of microstructures per unit area ρ. Figure 2: Effective normal force against displacement (see Fig.-1(C)) in the pull-off direction for a single microstructure and for various values of friction coefficient. 94 24th International Colloquium Tribology - January 2024 Mechanical Adhesion with Micropatterned Surfaces 3. Experimental section In order to evaluate the work of adhesion of patterned surfaces we opted for hexagonal patterns since the packing factor of this interlocking geometry is optimal and more robust against slight rotational tilts. The samples were fabricated through 3D printing techniques by means of a Formlabs 3B printer, using a stiff resin (E ~ 2.2-GPa) for the top approaching surface and a soft resin (E ~1-MPa) for bottom reference surface. The top sample was designed to be dome shaped in order to avoid boundary effects. A compliant shaft to hold the top sample was designed to allow slight adjustments needed for the interlocking of the surfaces. A custom made opto-tribometer was used to assess the performance of these surfaces. A typical test is divided into an approaching first part in which the samples are brought into contact by means of a linear actuator, when the force between the samples reaches a given threshold (2.5- N) the system holds the position allowing some relaxation of the polymer for 5-s, in the last part the linear actuator reverts its direction and a load cell measures the force during detachment. The tests were performed in dry and wet environment (surface completely submerged in distilled water). Figure 3: Adhesion tests performed on the same sample in dry and wet conditions. As expected, pull-off force is higher in dry than in wet conditions due to a higher friction coefficient. 4. Conclusions By introducing the actual material properties and geometrical parameters in our model, an equivalent work of adhesion of 1.51-N/ m was calculated in dry conditions and 0.35-N/ m in wet conditions while tests give us 1.78-N/ m and 1.31-N/ m respectively for dry and wet, showing that the model is robust when friction is high, this comes from the discrepancy between our model and the theory of JKR by which we calculated the work of adhesion from the pull-off force in the experiments. There is a noticeable increase in adhesion with respect to the smooth surfaces, even more in the case of smooth wet contact. This leads us to believe that an optimal set of parameters can be selected to further enhance the adhesion performances (radius of the spheres, overall shape of the interlocked structures, lattice shape, material stiffness and work of adhesion). A tailorable adhesion, effective even in wet conditions, can be disruptive in multiple fields ranging from medical suture and drug-delivery technologies, to industrial applications with adhesive-suppressing environmental conditions (which represent a major limitation to the adoption of van der Waals intermolecular interaction in micropatterned adhesives), to space applications where vacuum, dust, and other degrading factors could strongly impede adhesion. References [1] S. Baik, H. J. Lee, D. W. Kim, J. W. Kim, Y. Lee and C. Pang, “Bioinspired adhesi-ve architectures: from skin patch to integrated bioelectronics,” Advanced Materials, vol. 31, p. 1803309, 2019. [2] E. Gorb and S. Gorb, “Contact separati-on force of the fruit burrs in four plant species adapted to dispersal by mechanical interlo-cking,” Plant Physiology and Biochemistry, vol. 40, p. 373-381, 2002. [3] S. Y. Yang, E. D. O’Cearbhaill, G. C. Sisk, K. M. Park, W. K. Cho, M. Villiger, B. E. Bouma, B. Pomahac and J. M. 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