Sticky system modeling -Principle of sticky mechanism- Sangbae Kim

Motivation –Why duct tape works better than normal tape? –Why double sided tape with sponge in between two sides works better? –Why smooth surface is better for sticky tape? –Why rubber pad (tire, outsole of shoes) works better in grasping ground than stiffer material like steel? –Why soft polyurethane is sticky?

Compliance == Sticky? Chemical, atomic, capillary, electro static… etc… friction?

Sticky ? Hypothesis – ‘Stickiness’ depends primarily on compliance in structure. solid Stiffness Liquid ‘Sticky’

Term definition in Sticky system Sticky element –The smaller sticky element that composes upper scale sticky system. The sticky element produces force in the opposite direction of the force acting on the element (especially detaching force) like static friction force Sticky structure –The structure that configures sticky element to generate optimized sticky force. In other words, the structure that delivers force from the sticky element to upper scale structure. There are two different type of sticky structure. One is discrete structure, the other is continues one. Sticky force –Minimum external force that detaches sticky element or system.

Sticky system Sticky system class of Gecko feet Every branch can be sticky element and sticky system according to interested scale Surface roughness Toes Lamellae Branched Simple Nano-bumps Legs Performance Power point slide from POLY-PEDAL

Sticky system model Substrate Sticky element Sticky structure An element also has sticky system

Assumptions 1.Sticky structure is linear elastic material. 2.Sticky force of each sticky element is constant. – Which should be determined in smaller scale model 3.Elongation of sticky element is negligible. 4.There are no interactions between sticky elements –Discrete structure. 5.Substrate surface height follows Normal distribution. 6.Substrate and frame of sticky system (black line) are rigid. 7.Nominal length of sticky structure is long enough to comply. 8.Initial force is strong enough to engage every element.

Sticky system model. 1,3 : compression internal force 2 : Detached element ( if F i < k(Y i – h 0 ), where F i is sticky force of sticky element) 4: Zero internal force 5: Tensile force ( F i > k(Y i – h 0 )) F : external force k : stiffness of structure (modulus) F h0h0 YiYi k

Simulation Positive external force Negative external force Maximum sticky force Detaching point k=5 Plot between External force on sticky system vs distance from substrate

Elasticity k=20k=70 k=5 Plots in different stiffness in sticky structure

Maximum sticky force vs elasticity

Roughness Plot of relationship between Maximum sticky force and sigma(б) of surface height distribution

Engaging force In this simulation, we exclude the assumption No.8 that engaging force is strong enough so that we can estimate the ratio between engaging force and maximum sticky force

Application-claw design Sticky element Sticky structure - Compliance in y direction - Rigidity in x direction x y F Sticky force Driving force - We can modify sticky model for climbing changing angle of substrate and characteristic of sticky element.

Moment F

Passive engaging moment Force acting point More secure engaging

Small claw vs big claw Small sticky force of element - Less chance to get dull - Less chance to deform substrate Larger sticky force of element - More chance to get dull - More chance to deform substrate