Benjamin Porter1 Merce Pacios Pujado2 Harish Bhaskaran1

1, Univ of Oxford, Oxford, , United Kingdom
2, IREC Catalan Institute for Energy Research, Barcelona, , Spain

Controlling the assembly of nanoparticles into devices is a huge field, with approaches ranging from electrostatic, capillary-flows, DNA binding and optical tweezers all showing some efficacy in achieving nanoparticle assembly. Bringing these abilities into an industrial range hinges on building a thorough understanding the driving forces that nanoparticles heed. In our pursuit of more directed methods of nanoparticle assembly, we have discovered that electrostatically charged and hydrophilic nanoparticles will spontaneously assemble onto patterned regions of hydrophobic molecular monolayers on surfaces. We are able to show this is possible with single particle resolution in less than one minute, whilst demonstrating that assembly across more macroscopic length-scales can be achieved simultaneously1. We also find that the mechanism that drives this can be inhibited through control of the pH in the colloid. Whilst not fully understood, our working hypothesis for this process is that the water is dewetted from between the interface of the nanoparticle and the hydrophobic surface, in spite of the hydrophilic nature of the nanoparticle. This is unanticipated form of nanoparticle assembly, with previous findings determining that hydrophobic-hydrophilic interfaces would not result in breakdown of the hydrophobic barrier2, and could indicate a breakdown of common scientific understanding for nanoscale forces.
1. Porter, B. ~F., Pacios, M. & Bhaskaran, H. Hydrophobic-hydrophilic interactions drive rapid nanoparticle assembly. ArXiv e-prints (2017).
2. Zhang, X., Zhu, Y. & Granick, S. Hydrophobicity at a Janus interface. Science 295, 663–666 (2002).