(356i) Adhesion of Soft Materials to Wet, Compliant or Rough Substrates

Acrylic pressure sensitive adhesives (PSAs) are often used for medical tape applications on skin. Skin is a complex tissue: it is compliant, contains sweat glands (making surface wet or oily) and can also have varying hydrophobicity. PSAs based on skin applications have several tough demands to meet. For our investigation, we chose 2-Ethylhexyl acrylate-co-acrylic acid with 5%wt of acrylic acid as our PSA. I have performed mechanical testing through probe tack adhesion measurements with microscopic imaging, surface characterization through dynamic contact angle measurement and oscillatory rheology characterization of polymers.

In our previous study¹, we show that bonding of a rigid probe to PSA in water significantly reduces adhesion for both hydrophobic and hydrophilic probes. This reduction is much larger than expected from change in van der Waals forces alone. Contact between surfaces in water can lead to trapped water spots. These trapped water defects can change the mechanism of debonding from fingering and cavitation in air to external crack propagation in water. They also enhance the effect of water in adhesion – evident from higher adhesion on bonding in air and debonding in water.

Due to presence of ions in liquid media, variation of pH is expected. For example, trans epidermal water loss leads to skin surface pH 4-5,22 sweat has pH between 5 and 7, saliva has a pH 6.5-7.5,24 pH in the colon can reach up to 9,25 and soapy water is usually alkaline with a pH 10-12. Investigation of pH effect is done by comparing acrylic PSA with 5wt% acrylic acid (5%AA) versus 0wt% acrylic acid (0%AA). We find deprotonation of acrylic acid causes change in surface energy of 5%AA PSA leading to reduced adhesion at higher pH.²

The compliance of substrate can also affect adhesion and observed instabilities. We investigate the effect of soft probe on adhesion and interfacial instabilities of a model viscoelastic polymer. We observe suppression of fingering instabilities by softer probes.

Roughness of substrate can also play an important role on adhesion of a soft material. Expected effect of roughness would be lower adhesion of rougher materials due to lesser contact area. However, our preliminary measurements reveal higher adhesion for rougher, soft materials. We will present experimental data investigating the role of surface stress in adhesion of rough and soft material systematically.

* Work described here is primarily by Preetika Karnal, but also has contributions from Professor Joelle Frechette, Professor Anand Jagota, Dr. Carlos Barrios, Stefan Gryska, Paul Roberts, Anushka Jha, Hanqi Wen, Dr. Yumo Wang and Courtney King.

1. Karnal, P.; Roberts, P.; Gryska, S.; King, C.; Barrios, C.; Frechette, J., Importance of substrate functionality on the adhesion and debonding of a pressure sensitive adhesive under water. ACS Applied Materials & Interfaces 2017.
2. Karnal, P.; Jha, A.; Wen, H.; Gryska, S.; Barrios, C.; Frechette, J., Contribution of Surface Energy to pH-Dependent Underwater Adhesion of an Acrylic Pressure-Sensitive Adhesive. Langmuir 2019, 35 (15), 5151-5161.

Teaching Interests

I would enjoy the opportunity to teach undergraduate or graduate course in Chemical Engineering preferably on topics like transport phenomena, interfacial science or polymers.