12342
Langmuir 2005, 21, 12342-12347
Colloid Probe AFM Investigation of the Influence of Cross-Linking on the Interaction Behavior and Nano-Rheology of Colloidal Droplets Graeme Gillies† and Clive A. Prestidge* Ian Wark Research Institute, The ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, South Australia, 5095, Australia Received June 8, 2005. In Final Form: August 31, 2005 The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hardsphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly crosslinked droplets, the extent of deformation is controlled by the droplet’s bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.
Introduction Over the past half decade, the colloid probe AFM technique1-3 has been increasingly used to study interaction forces of deformable systems, including polymer microspheres4-8 and microcapsules,9,10 biological cells,11-14 droplets,15-19 and bubbles.20-22 From these reported * To whom correspondence should be addressed. † Current address: Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. (1) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (2) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (3) Butt, H.-J. Biophys. J. 1991, 60, 1438. (4) Reitsma, M.; Craig, V. S. J.; Biggs, S. R. J. Adhes. 2000, 74, 125. (5) Schmitt, F.-J.; Ederth, T.; Weidenhammer, P.; Claesson, P.; Jacobasch, H.-J. J. Adhes. Sci. Technol. 1999, 13, 79. (6) Vakarelski, I.; Toritani, A.; Nakayama, M.; Higashitani, K. Langmuir 2003, 19, 110. (7) Vakarelski, I.; Toritani, A.; Nakayama, M.; Higashitani, K. Langmuir 2001, 17, 4739. (8) Rutland, M. W.; Tyrrell, J. W. G.; Attard, P. J. Adhes. Sci. Technol. 2004, 18, 1199. (9) Lulevich, V. V.; Vinogradova, O. I. Langmuir 2004, 20, 2874. (10) Lulevich, V. V.; Radtchenko, I. L.; Sukhorukov, G. B.; Vinogradova, O. I. J. Phys. Chem. B 2003, 107, 2735. (11) Velegol, S. B.; Logan, B. E. Langmuir 2002, 18, 5256. (12) Dvorak, J. A.; Nagao, E. Exp. Cell. Res. 1998, 242, 69. (13) Wu, H. W.; Kuhn, T.; Moy, V. T. Scanning 1998, 20, 389. (14) A-Hassan, E.; Heinz, W. F.; Antonik, M. D.; D’Costa, N. P.; Nageswaran, S.; Schoenenberger, C.-A.; Hoh, J. H. Biophys. J. 1998, 74, 1564. (15) Hartley, P. G.; Griesser, F.; Mulvaney, P.; Stevens, G. W. Langmuir 1999, 15, 7882. (16) Aston, D. E.; Berg, J. C. J. Colloid Interface Sci. 2001, 235, 162. (17) Nespolo, S. A.; Chan, D. Y. C.; Grieser, F.; Hartley, P. G.; Stevens, G. W. Langmuir 2003, 19, 2124. (18) Dagastine, R. R.; Prieve, D. C.; White, L. R. J. Colloid Interface Sci. 2004, 269, 84. (19) Dagastine, R. R.; Prieve, D. C.; White, L. R. J. Colloid Interface Sci. 2002, 247, 310. (20) Preuss, M.; Butt, H.-J. Langmuir 1998, 14, 3164.
investigations, it is interesting to note that deformation mechanisms are highly contrasting and dependent on the physical nature of the experimental system: for polymer microspheres and cellulose particles, deformation is independently controlled by the elastic modulus; the deformation of cells and ‘filled’ microcapsules is controlled by the combined influence of the excess osmotic pressure inside them and the shell or membrane rigidity,9 whereas droplet and bubble deformation is governed by the interfacial tension.15,17 Notable investigations that make use of experimental systems with varying deformability include those of Filip et al.,23 Vakarelski et al.,6,7 and Rutland et al.8 Filip et al.23 have shown for water in oil emulsion droplets that inclusion of >15% gelatine into the water droplets results in deformation behavior dominated by the bulk elastic modulus, whereas at
