(552h) Effect of Amphiphilicity and Janus Cap Orientation on Janus Particles at an Air-Water Interface

Well-defined anisotropic particles present unique opportunities to study directed assembly. More specifically, Janus particles allow one to precisely control the amphiphilicity of the particle and the surface wetting properties on each hemisphere. Interfacial assembly of anisotropic particles is of particular interest as these confined particles can be used to template microstructures in a bottom up fashion. The fluid-fluid boundaries can trap colloids, orienting particles in the direction normal to the surface and patterning particles in the plane of the interface⁽¹⁾. The ability to pattern particles at interfaces has a multitude of applications such as achieving optical properties, making antireflective surfaces, improving the resolution of E-paper displays and creating water repellant materials⁽²⁾. Furthermore, anisotropic particles at interfaces can be applied as emulsion stabilizers and catalysts.⁽³⁾ In all of these applications, the amphiphilicity of Janus particles plays an important role in influencing particle orientation, stability and assembly at the interface. Additionally, understanding the free-energy landscape of Janus particles at an interface can help explain and predict particle behavior under different circumstances.

The work presented studies gold capped silica Janus particles with varying degrees of amphiphilicity. A range of contact angles of the gold surface is achieved using modification with alkane thiols of various chain lengths. Colloidal AFM experiments have been conducted to elucidate how orientation and amphiphilicity of Janus particles influences their behavior at an air-water interface. In this work, Janus particles at various orientations are attached to an AFM cantilever and force is measured as the particle interacts with and is wet by the air-water interface of a surface immobilized bubble. These experiments allow for quantification of surface forces, adhesion, contact angle, and give insight as to how the energy landscape of the particle changes with orientation and amphiphilicity. Such phenomena can be compared to theory and connected to behavior of particle-laden interfaces, studied on a Langmuir trough, in order to connect microscopic and macroscopic behavior.