(86d) Adhesion Forces Between Individual Oxide Nanoparticles in Gas-Phase Processes

Nanoparticles serve as the building block in an increasing amount of advanced materials and devices: they are often coated on a surface or dispersed in a matrix. Practical examples are fluidization, coating, or mixing of nanoparticles. In order to do this properly, it is crucial to have good control over the contact forces between the nanoparticles. However, in the scale down to the nano-range, contact forces are relatively strong and determine the interplay of individual nanoparticles. Hence, a fundamental understanding of the contact forces between single nanoparticles is necessary to achieve highly efficient processes and products.

Here we present a comprehensive force spectroscopy study of various metal oxide nanopowders under different gas-phase conditions to preserve an understanding of adhesion forces between individual nanoparticles. Therefore, highly porous agglomerates in the size of hundreds of microns were fixed onto a flat substrate with double sided tape. These spread agglomerates were penetrated with an AFM-tip. During retracting the tip a chain of nanoparticles unfolds between the tip and the agglomerate which finally detaches at a contact between two single nanoparticles [1]. By investigating hundreds of these measurements we collected force informationâ??s for most relevant oxides such as TiO₂, Al₂O₃, and SiO₂. These quantitative analysis was verified with a special AFM-TEM set-up recording the unfolding process between the tip and the agglomerate in situ[2]. For a comprehensive analysis we measured the adhesion forces in a humidity range from 0-90 %, in vacuum, and in isopropanol enriched nitrogen. The results of the experimental studies suggest that the adhesion forces between individual nanoparticles (sizes of 10s of nanometers) cannot only be interpreted by state-of-the-art continuum theories such as van der Waals or capillary forces. Rather the molecular nature of the physisorbed water molecules strongly influences the particle-particle interaction [3]. An extensive surface characterization of the investigated nanoparticles by FTIR, TGA, and TEM reveals that a noticeable amount of physisorbed water molecules is always present on the surface which only can be removed by using high temperatures. The molecules lead to the formation of a water meniscus between contacting nanoparticles in which the molecules take discrete positions: it cannot be treated as a continuous meniscus. These findings were confirmed by all-atom MD simulations of two interacting nanoparticles. Furthermore, these structuring is strongly influenced by the surface chemistry of the nanoparticles as shown experimentally by using surface modificated nanoparticles. Hydrophobic nanoparticles show decreased adhesion forces which we related to the weaker interaction of the organic surface groups with the always present water molecules.

Summarizing, we will show that adhesion forces between individual oxide nanoparticles can be described by a combination of continuum (capillary) and non-continuum (structuring) forces which has a large impact on the handling and processing of nanoparticles.

[1] S. Salameh, J. Schneider, J. Laube, A. Alessandrini, P. Facci, J.W. Seo, L. Colmbi Ciacchi, L. Mädler, Adhesion Mechanisms of the Contact Interface of TiO₂Nanoparticles in Films and Aggregates, Langmuir, 28 (2012) 11457-11464.

[2] S. Salameh, R. Scholz, J.W. Seo, L. Mädler, Contact behavior of size fractionated TiO₂nanoparticle agglomerates and aggregates, Powder Technol., 256 (2014) 345-351.

[3] J. Laube, S. Salameh, M. Kappl, L. Mädler, L. Colombi Ciacchi, Contact Forces between TiO₂ Nanoparticles Governed by an Interplay of Adsorbed Water Layers and Roughness, Langmuir, 31 (2015) 11288-11295.