Sticking to surfaces

Examples of natural adhesion- temporary to permanent

Some adhesives are inspired by nature

https://ninithi.wordpress.com/2015/09/11/spiderman-gloves-more-nanotechnology-enables-gecko-pads/

Other adhesion examples

http://4.bp.blogspot.com/-pEB7Z8jM2ws/UlmeaOVtf6I/AAAAAAAAQCk/CLStC-DkEK0/s1600/DSCN6334+-+Copy.JPG

https://www.e-know.ca/wp-content/uploads/2015/10/Burr-head-dog.jpg

Adhesion

Liquid Solid

Intimate contact Keeping things together

MonomersMeltsSolutionsSuspensions

EnvironmentApplied stress

Theories to why things “stick”

1. Mechanical Interlock– Tooth fillings to velcro2. Inter-diffusion– Plastic pipe welding3. Secondary interactions– Intermolecular forces: London dispersion,

dipole-dipole interactions, hydrogen bonds, specific attractions 4. Electrostatic charge model: permanent charge (electrostatically

layered films)5. Covalent linkages: chemical bond formation with the adherend i.e.

coupling agents

Why things don’t “stick”

All molecules are attracted to each other– sometimes self attraction is much greater than attraction to adherend

Surface tension (liquid) Surface free energy (solid)Work to create additional surface areaisothermally and reversibly by unit amount

γ0

Water --- 72.8 mN/mBenzene--- 28.9 mN/mHexane --- 18.4 mN/mAcetic acid ---27.6 mN/mAcetone ---23.7 mN/mEthanol --- 22.3 mN/mOctanol --- 27.5 mN/m

http://www.educationquizzes.com/library/KS1-Science/Blowing-Bubbles/Blowing-Bubbles-Main.jpg

Cohesion and work of adhesion

• Work of cohesion• Work to pull apart a column of water• Work of Cohesion = 2X surface energy of liquid• Wc= 2γL

• Work of adhesion• Spreading (work of adhesion must be greater than the work of cohesion)• Dupre’equation

• Wa=γA+γB -γAB

• Work of Adhesion = γgL x (1 +cos θ) (theta equals contact angle of droplet)

Probing surface chemistry using the contact angle method

http://www.attension.com/$2/sessiledrop3angles-1.jpg

Surface issues with adhesion– the good and bad• Roughness

• Macroscale roughness to nanoscale roughness

• Contamination (surface different from bulk)• Arising from “air”

• Aging• Contamination as well as migration of extractives

Native lumens also can be textured “warty layers” etc.

Engineered Wettability in Tree CapillariesMika M. Kohonen*Langmuir 2006 22 (7), 3148-3153

Singh et al. High variability in the thickness of the S3 layer in pinusradiate tracheids. Holzforschung, 2002 56:111-116.

All wood surfaces are variable at the micro down to the molecular level

www.branhamcorp.com http://www.oakwoodveneer.com/tips/cuts.html

Wood surface impacted by processing history

Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002

Wettability changes as a function of heating temperature

Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002

Wood surfaces are not static either

From “effects of surface ageing on wettability, surface energy, and adhesion of wood” by M. Gindland co-workers published in Holz RohWerkst (2004) 62:273-280

Surface chemistry correlates with adhesive performance – “can’t bond a bad surface”

Milan Sernek, PhD Dissertation at Virginia Tech, “Comparative analysis of inactivated wood surfaces” 2002