Quantitative Surface Free Energy measurements in Electrets using Micro-Colloid Probe Pairs
Ehtsham-Ul Haq1, Yongliang Zhang, Stanislav Leesment2, Ning Liu1, Claude Becker4, Syed A. M. Tofail1 and Christophe Silien1
Ehtsham.U.Haq@ul.ie
1Department of Physics, and Bernal Institute, University of Limerick, V94 T9PX, Limerick, Ireland
2Spectrum Instruments Ltd. Stewart House, National Technological Park, Limerick, Ireland
3Bernal Institute, School of Engineering, University of Limerick,, V94 T9PX, Limerick, Irelan
4Funcoats SA, Technoport 4B - rue du commerce Foetz, Luxembourg

Abstract missing:

Introduction

Surface charge and free energy of surfaces are important in electrets as well as it is critical to control wetting, adhesion, and friction properties in tribology, photonics, scaffold materials, particles for drug delivery and bio-coatings [1]. Development and validation of standardized, quantitative methods to reliably measure surface properties at relevant length scales remains an industry requirement to account for important contact phenomena such as adhesion and friction [4]. Here, a protocol for quantitative measurements and mapping of the surface free energy is presented, which reliably connects measurements at the micrometer scale to conventional measurements at the macroscopic scale [5].
Results and Discussion
A microscale Atomic Force microscopy (AFM) colloidal probes attached onto a cantilever, the bending of which allows measuring the total force of interaction between the tip and the surface of a specimen of interest is exploited. In general, quantitative SFE measurement with AFM is hampered by relatively poor precision due to unknown tip relevant properties (e.g., geometry, contact geometry, surface physical chemistry, etc.) and, more importantly, the reliance on contact models to compute SFE from AFM observed metrics (pull-off force, snap-in, etc.). The study resulted in the development of a methodology that employs two colloidal probes (10 microns silica colloid and 10 microns polystyrene (PS) colloid, contact area: ca. 1 micron diameter). The colloids are complementary, and they operate in tandem to refine the SFE outcome. A series of laterally homogenous calibration specimens with contact-angle SFEs are then measured in order to calibrate a regression of AFM-derived main components onto the corresponding contact-angle SFEs. This indicates that the SFE is calculated using a single model derived from the contact angles (Owen Wendt's model) (Figure 1). The feasibility of the relevance of this SFE analysis protocol to materials of biological origin is demonstrated on drop casted Methionine peptide crystals from water and ethanol solution on a clean ITO coated glass substrate.

Figure 1 (a) PC2 versus PC1 scatter plots for the training set with PS and Silica colloids. The data for sh-Si are shown in black, HOPG in red, silica in green, silicon in blue, and mica in cyan. (b) Second-order regression of the PC1, PC2, and PC3 values on the CA-derived SFE values for the testing set with PS and Silica colloid.

Conclusions
A method is described for quantitatively measuring and mapping the surface free energy. The method is particularly useful for bioelectrets, such as hydroxyapatite, Polyvinylidene fluoride (PVDF) and biological crystals where no other reliable method is available for SFE characterization.

References
[1] E. Haq et al. Quantitative surface free energy with micro-colloid probe pairs. RSC advances (2023).
Acknowledgements
European Union's Horizon 2020 research and innovation program, OYSTER (Open characterisation and modelling environment to drive innovation in advanced nanoarchitectured and bio-inspired hard/soft interfaces) under grant agreement No 760827.