Adhesion and Detachment Characteristics of Soft Adhesive Systems: from pressure-sensitivesensitive adhesive tapes to gecko hairy foot pads Boxin Zhao, Ph.D. Assistant Professor Waterloo Institute for Nanotechnology, Department of Chemical Engineering, University of Waterloo, Canada Symposium, May 1
Adhesion is a Fundamental Phenomenon in Nature St Croix, US Virgin Islands Gecko climbing on bamboo surfaces Spider Web Dew drops adhering to a spider web 2
Adhesion is also Essential to Engineering and Future Innovations Adhesive tapes & labels 20 m Paper fiber network 10nm Adhesive pads 1mm Polymer adhesive shield Mico/nano particles Polymers are Good Adhesives StickyBot, Stanford Univ, 2006 3
Molecular Adhesion is Universal Intermolecular attractive Interaction (Van der Waals forces) Johannes Diderik van der Waals The Nobel Prize in Physics 1910 1 1 Unit area < 10nm 2 St Croix, US Virgin Islands Two smooth surfaces leap into contact at nanometer (10-9 m) distance Human hair ~ 100 micrometer in diameter 4
Practical Adhesion is Complex Surface roughness reduces adhesion Surface deformation in detachment Cracks The adhesion and detachment mechanisms matter 5
Outline PSA tape The adhesives/paper interactions Recent research Viscoelastic thin coating films Future Research Gecko adhesive system Biomimetic or Bio-inspired Adhesion and Smart Adhesives 6
Overall Objectives To identify and characterize the behaviors of soft (synthetic and biological) adhesive surfaces and associated micromechanical properties To develop new concepts, approaches and techniques to tune adhesion and make smart adhesives. Today: to highlight key research findings 7
Pressure-sensitive sensitive Adhesives/Paper Interactions 8
What are Pressure-sensitive Adhesives? PSAs are materials which adhere under a light pressure. PSAs are polymeric and have a property called viscoelastic. They behave like liquid in bonding while fracture like solid in debonding. Face material Adhesive ~ 50 m Backing material ~50 m Coating layers CH 2 CH (Release / bonding Agent) C O R O OH C CH 2 CH O CH 2 CH Acrylic PSA C O R O 9
PSAs Used in Papermaking Splicing tape Flying-splice in papermaking mills Performance requirements: (1) Instant adhesion (2) Strong joint strength for survival in further processing at ~ 60km/hr (3) Repulpable in recycling Occasional Failure costs millions $$$ Research questions: What are the fracture mechanisms? How to make stronger adhesive bonds? 10
Peeling Adhesion Analysis Easy to perform Providing information on both paper and adhesive tape It involves complex mechanical effects Peeling angle Bending curvature 11
Wheel Peeling Tester Video Camera Constant peeling angle = 90 PSA Tape F d wheel rotating ti Paper Wheel We fixed d = 0 in most of our measurements 12
Peel Forces and Interfacial Phenomena Peel For rce, N/mm 0.6 05 0.5 0.4 0.3 0.2 0.1 0 Typical peeling curves Peak Force Mixed Failure Interfacial Interfaicial Failure Paper Failure 400mm/min 100mm/min 0 10 20 30 40 50 Peel Distance, mm Tape Paper Delamination Interfacial failure Adhesive fibrils Paper failure 13 Fibers do not break in delamination.
Both Adhesion Forces and Failure Modes are Functions of Velocity eeling Fo orce Max. P PSA cohesive failure In logarithmic scales Contact pressure & time Surface energy & roughness Paper failure interfiber bonding strength V c Peeling Velocity Identified a critical velocity, V c for the transition of failure modes Established the link between paper, tape properties, and adhesion performance. Surface energy is determined by surface chemistry Zhao, Pelton, Tappi, 2004 Zhao, Anderson, Banks, Pelton, J. Adhesion Sci. Technol. 2003, 2004 14
Using Tape-peeling as a Measure of Paper Surface Strength The peeling method Industrial standard method This method cost less than industrial methods; it is adopted by the Australian Pulp and Paper Institute. Zhao and Pelton, Tappi, 2004 15
O Adding Polyelectrolytes (PE) to Tune Interfiber Adhesion Strength Interfiber-bonds Polymer chain Fiber surface negatively-charged Adsorption Water removal H 2 H 2 C H H C CH - C C 2 OH O H 2 C CH O H 2 NN+ HO OH H 3 C CH 3 Adhesion force 10nm n Adhesion Hydrophilic cellulose negative charged Hydrophilic PE to enhance adhesion Hydrophobic PE to reduce adhesion 16
Summary Adhesives are highly deformed and form fibrils. The adhesion forces increase and failure modes change as peeling velocity increases. The max adhesive/paper joint strength is determined by paper surface strength. This finding resulted in a simple approach to measure paper surface strength. Interfiber adhesion strength th can be tuned by adding polyelectrolytes. 17
Dynamic Adhesion and Fracture of Thin Coating Films: Solid- and Liquid-like Failure 18
Failure Mechanisms - Two Extreme Scenarios Snapping of a liquid bridge liquid. Behaviors of soft materials is far less understood Brittle fracture of solid solid e.g. water, viscosity of 10-3 Pa. S e.g. ice, viscosity of 10 11 Pa.S 19
Research Objectives To identify and characterize: the differences between liquid- and solid-likelike failure mechanisms of micro/nano thin films Molecular interaction, surface deformation and instabilities in adhesion and subsequent separation 20
Sugar Viscosity as a Function of Temperature Simples sugars are ideal materials to study the solid- and liquid-like behaviors 21
Sugar Films Coated onto Mica Surfaces Mica surface Amorphous sugar coating ~0.05µm 05µm Mica surface Three typical experimental temperatures in N 2 atmospheres Glassy state at 23 C Viscoelastic state at 40 C Viscous fluid state at 75 C AFM imaging of sugar surface Image RMS = 0.537nm 22
Using SFA and FECO to Study Adhesion Failure Mechanisms SFA: Surface Forces Apparatus FECO: Fringes of Equal Chromatic Order - thin film interference patterns light Microscopy imaging F 2a Adhesive contact Top view Newton s ring FECO fringes Spectrometer 50nm 2a SFA 2000 Tabor, Winterton, Israelachvili,1970 Wavelength (Å) 23
Using the JKR-theory as An Analytical Tool 2a in Jump-in 2a out Steady-state peeling Jump out - an abrupt peeling, 2a, a eter Contact diam Classic adhesion theory ( equilibrium and elastic system) JKR theory - reversible process JKR - plot Jump out F ad Unloading Jump in loading, F Compressive load F ad 3 R Johnson, Kendall, Roberts (1971) Proc R Soc London Ser A 324:301 313. Surface energy 24
Contact Behavior of Sugar Surfaces at 23 C C, Viscosity of 10 14 Pa.S Contact diame eter ( m) 80 Jump out 40 46 5mJ / m 2 Unloading 120 Loading 0 JKR theory Glucose 23 C -50-25 0 25 50 Compressive load (mn) Adhesion Hysteresis - time effects Crack initiation Contact area Jump-out Propagation (<30ms) Brittle fracture/cracking 25
Time Effects Contac ct diameter ( m m) (c) (b) Jump out (a) 150 100 50 (A) Glucose 23 C 0-75 -50-25 0 25 50 75 Compressive load (mn) Contact time: a < b < c F/R (N/m) 8 6 1 m/s 4 2 0 0.2 m/s (B) Glucose 23 C 0 20 40 60 80 100 Contact time (minutes) Separation velocity Air Glucose molecules rearrangements at the interface - a dynamic process 26
Adhesion and Coalescence of Viscous Fluid Surfaces at 75 C(10 3 Pa.S) 20µm Coalescence occurs right after adhesive contact Contact area scales with time Radius Radius of cont of tact meniucs area ( m) ( m) 160 120 80 40 0 a ~ t 0.2 0 200 400 600 Transient surface patterns during fluid-fluid coalescence Zeng, Zhao, Tian, Tirrell, Leal and Israelachvili, 2006 Time (s) 27
Detachment of Viscous Sugar Surfaces Viscous fingerings due to the Saffman-Taylor instability d a ~50nm ~200 m 20 m a d 4000 Instability during the peeling of adhesive tape Glass su urface Adhesi ive tape F. Frankel, G. M. Whitesides, On the Surface of Things, 1997 Viscous fingerings consume a large amount of energy, giving a strong adhesive bond. 28
Solid-like sharp tips (high local stress) IP R 20 09 Evolution of Interface Ripples/waves in Detachment video demo a slow-down process This may be due to the lateral acceleration of fluid during its normal separation. Liquid-like rounded fingers (low local stress) Cavitations Co-existence of sharp tips and round fingers were observed for the first time, suggesting a 29 unifying theory.
Summary The fracture of two adhered surface was manifested by crack nucleation and propagation at one extreme and the snapping of a liquid bridge at the other The fracture of two adhered viscoelastic surfaces was manifested by rounded fingers Practical Implications Cavitations and fingerings consume a large amount of energy, resulting in a strong adhesive bond. Adhesion can tuned by adjusting material viscosity. 30
Understanding Gecko Adhesive System learn from nature 31
Gecko a Super Climber Tokay gecko on walls House gecko on ceilings John Bokma @2007 There are about 850 gecko species. We focus on Tokay gecko, the largest species Marbled Gecko Photo Courtesy Ben Moulton Hawaiian Gecko St Croix, US Virgin Islands Quincy Dein Photography 32
What is Known about Gecko Adhesion A Spatulae ( - keratin nanostructures) behave like adhesive tape. Adhesion via intermolecular van der Waal forces 100-1000 spatulae/setae ~14,400 setae/mm 2 B 100 nm 200 nm 10 nm 33
Recent Research and Challenges Geim, Nature, 2003 Many research on the fibrillar surfaces (varied aspect-ratio, shape) 2 m Physical characterization of gecko attachment and detachment. Design of responsive surfaces for smart adhesives and robotic applications StickyBot, Stanford Univ, 2006 UCSB team Kim, 2007 34
Gecko Attachment and Detachment - Peeling Mechanism Walking gecko on walls FF F rgy, F/( b) Peeling Ene Gecko engages attachment at small pulling angles while detachment at large angles 1000 100 10 1 0.1 F b ( 1 cos ) 0 40 80 120 160 Peeling angle, Detaching gecko foot 35
Gecko Attachment and Detachment - Peeling Mechanism Walking gecko on walls Friction Adhesion FF F rgy, F/( b) Peeling Ene Gecko engages attachment at small pulling angles while detachment at large angles 1000 100 10 1 0.1 F b ( 1 cos ) 0 40 80 120 160 Peeling angle, Surface features of the gecko setal arrays Detaching gecko foot 36
Gecko Foot Pad vs Adhesive Tape A Strong -keratin, Soft polymer E~10 9 Pa E~10 5 Pa Build-in micro/nano fibrillar structures Stress-induced adhesive fibrills Fibrillar structures consume a large amount energy in detachment, resulting in high adhesion strength 37
Contact Dynamics (Adhesion and Friction) Measurements Gi Grippingi Releasing High Adhesion High Friction High Repulsion Low Friction Surface Forces Apparatus Gecko setal arrays are structurally anisotropic, exhibiting strong directional adhesion and friction properties. 38
Frictio on forces s, F ±v (mn N) 16 12 8 4 Gripping direction Adhesive friction behavior JKR F +v Releasing direction Load - dependent friction behavior Amontons friction law = F -v / L -v = 0.25 0-8 -4 0 4 8 L Adhesion Repulsion +v L -v Normal force, L ±v (mn) To mimic gecko adhesive pads and functionalities, anisotropic curved structure is essential. Zhao, Israelachvili, et al., Langmuir, 2007 39
Summary Gecko foot pads behave like adhesive tape while its robust and responsive adhesion arises from the build-in micro/nano-sized fibrillar structures. Many things are still unknown, e.g., the formation of gecko fibrils. This suggests a new strategy to design and tune adhesion by surface patterning. A 40
Current and Future Research Biomimetic Adhesion and Smart (responsive, adaptable) Adhesive Devices 41
Biomimetic studies for responsive and adaptable materials Nature Science Fabrication Application?? Responsive and adaptable to external, both chemical and mechanical, stresses
NON-Responsive surface Climbing Velcro man Video demo Tokay gecko on walls 5mm
Overall Research Objectives As future technological innovations gear towards miniaturizing machines and maximizing performance density, our challenges as engineers and scientists become our ability to build micro- and nano-machines and understand phenomena at a scale we normally do not deal with. Focus on polymeric materials for both mechanical and biological applications. 44
Biological Fibrillar Adhesive Structures A Fibrillar micro/nano structures is characteristic ti of biological i l attachment devices, which are natural Post-it Note (Materials Today, 2004) 45
Fabrication of micro polymer pillars (i) Microfabrication of silicon masters for molding (ii) PDMS mold fabricated using silicon masters (iii) Polymer micro-pillar structures fabricated using PDMS mold Key design factors: Number density, Aspect ratio, Mechanical strength, Surface chemistry In collaboration with Dr. Israelachvili and Dr. Turner group at UCSB 46
Fabrication of micro polymer pillars with tailored properties Water droplet Micro-structured surface In collaboration with Dr. Alex Penlidis and Dr. Neil McManus S.-H. Zhu, N.T. McManus, C. Tzoganakis, A. Penlidis, 2007 47
Fabricating Curved Structures Gecko directional adhesion Tilted PDMS mold To mimic this properties, we fabricate curved pillars. Curved micro-pillars In collaboration with Dr. Turner group at UCSB and Dr. Pesika group at Tulane University 48
A Summary Gecko-like micro/nano structured materials FABRICATION OF BIOMIMETIC STRUCTURES AT SMALL SCALES FOR RESPONSIVE AND ADAPTABLE MATERIALS APPLICATIONS PROF. BOXIN ZHAO CHEMICAL ENGINEERING, U WATERLOO Surface forces and micro/nano tirbological studies Micro/nano porous hydrogels, artificial cartilage, and joint lubrication
Acknowledgements UCSB Dr. Jacob Israelachvili Dr. Noshir Pesika Dr. Hongbo Zeng Dr. Yu Tian Dr. Kenny Rosenberg Dr. Patricia McGuiggan Dr. Mathew Tirrell Dr. Gary Leal Collaborators Dr. Kellar Autumn (Lewis & Clark College, Oregon, USA) Dr. Kim Turner (Mechanical Department, UCSB ) McMaster University Dr. Robert Pelton Dr. Shiping Zhu Dr. John MacGregor Dr. Honglu Yu Dr. An-chang Shi 50