A clean surface with increased surface wettability is oftentimes beneficial to promote adhesion and enhance bonding to other surfaces. This article discusses the benefits of plasma treatment in altering surface wettability characteristics for adhesion and other applications, plasma processing guidelines, and examples of contact angle measurements on plasma-treated materials.

For references citing the use of our plasma cleaners in adhesion and wettability applications, see the Surface Adhesion and Surface Wettability categories in our Technical Library.

Benefits of Plasma Cleaning

Plasma cleaning removes organic contaminants by chemical reaction (air or O2 plasma) or physical ablation (Ar plasma). Plasma treatment also introduces chemical functional groups (carbonyl, carboxyl, hydroxyl) on the surface, rendering most surfaces hydrophilic. This is typically observed as a decrease in water contact angle and increased wettability [Figure 1 and Figure 2]. Increased wettability prepares the surface for subsequent processing (e.g. film deposition or adsorption of molecules) by improving surface coverage and spreading of coatings and enhancing adhesion between two surfaces.
Figure 1. Water drop contact angle measurement on 316L stainless steel (a) as received, (b) after chemical clean (ultrasonication in 70% ethanol, acetone, and 40% nitric acid), and (c) after chemical clean and O 2 plasma treatment using a Harrick Plasma cleaner. Data from Mahapatro, A., D. M. Johnson, D. N. Patel, M. D. Feldman, A. A. Ayon, C. M. Agrawal. “Surface Modification of Functional Self-Assembled Monolayers on 316L Stainless Steel Via Lipase Catalysis.” Langmuir (2006) 22: 901-905.
Figure 2. Water droplet contact angle measurement on ultrahigh molecular weight polyethylene (UHMWPE) as a function of O 2 plasma treatment time using a Harrick Plasma cleaner. Data from Widmer, M. R., M. Heuberger, J. Vörös, N. D. Spencer. “Influence of Polymer Surface Chemistry on Frictional Properties under Protein-Lubrication Conditions: Implications for Hip-Implant Design.” Tribol. Lett. (2001) 10: 111-116.

Example Applications

Surfaces can be plasma treated to modify surface chemistry without affecting the bulk properties of the material. As such, plasma treatment can be applied to a wide variety of materials as well as complex surface geometries. Below are examples applications and samples that have been treated in our plasma instruments:

  • Clean gold surfaces prior to self-assembly experiments
  • Activate electron microscopy (EM) grids prior to applying samples for imaging
  • Oxidize titanium to enhance biocompatibility as a medical implant material
  • Treat biomaterial surfaces and polymer scaffolds to enhance cell attachment and spreading for cell culturing
  • Plasma treat fibers to improve adhesion to matrix in fiber-reinforced composite materials
  • Clean AFM cantilever tips to study adhesion characteristics of dissimilar materials by mechanical testing or AFM force measurements
  • Activate carbon surfaces to promote attachment of silver (Ag) nanoparticles, for use as an antibacterial medium and for water disinfection

Processing Methods

Air or oxygen (O2) gas is typically used for plasma cleaning and surface activation. An air or O2 plasma removes organic contaminants by chemical reaction with highly reactive oxygen radicals and ablation by energetic oxygen ions. The plasma also promotes hydroxylation (OH groups) on the surface, rendering the surface more hydrophilic and increasing surface wettability.

Alternatively, argon plasma may be preferred for surface activation to minimize further oxidation of surfaces (e.g. metals). Argon plasma cleans by ion bombardment and physical ablation of contaminants off the surface and can also increase surface hydrophilicity by reaction of the plasma-activated surface upon exposure to ambient air.

For applications that are sensitive to potential contamination from trace impurities in borosilicate glass, a quartz chamber is recommended over the standard Pyrex chamber.

Below are suggested process conditions for plasma cleaning in a Harrick Plasma cleaner (some experimentation may be required to determine optimal process conditions):

  • Pressure: 500 mTorr to 1 Torr
  • RF power: Typically HIGH
  • Process time: 1-3 minutes
  • Low RF power may be used to minimize surface roughening; the process time may require adjustment to compensate for the lower power
  • Surfaces should be used immediately after plasma treatment; plasma-treated surfaces may recover their untreated surface characteristics with prolonged exposure to air.

Harrick Plasma is a leading supplier of plasma equipment to the research community. We have been providing quality tabletop plasma devices specifically designed for laboratory and R&D use for over 30 years.