Sensing and Visualizing Nanoparticle Distribution and Flow on Model Rough Surfaces

Understanding the interactions between nanoparticles and rough surfaces is very critical to predicting their flow and transport at a larger scale. In this work, we report an innovative label-free approach to sense and visualize the flow and distribution of Titanium dioxide nanoparticles (nTiO2) on engineered porous surfaces with nanoscale roughness, i.e., slanted columnar thin films (SCTFs). This approach uses principles of Generalized Ellipsometry (GE), a highly accurate, non-destructive, and non-invasive optical method, which measures the change in polarization when light reflects off or transmits through a sample, before and after the deposition of nTiO2. Here, SCTFs served as an anisotropic substrate for GE measurements, as well as a surface with a controlled roughness height.

Nanoliter volumes of nTiO2 were dispersed into SCTF surfaces by controlled piezoelectric plotting. Attachment of nTiO2 on the surface of SCTFs changed the optical property of SCTFs, which was detected using a GE based instrument, anisotropy contrast optical microscope (ACOM). An anisotropic effective medium model was applied to quantitatively analyze ACOM images of SCTF surfaces, which provided the mass distribution of nTiO2. nTiO2 mass measured by ACOM was in good agreement with the known amount of nTiO2 mass dispersed. Detection of few pico-gram of nanoparticle mass by an individual pixel of 7×7-micrometer-square was demonstrated. Further, a glass-microfluidic channel with SCTF embedded was developed. The areal mass density of attached nTiO2 on SCTF surfaces as they flow through the channel under various flow rates was quantitatively measured in-situ. At the end of the experiment, the distribution of attached nTiO2 on the SCTF surface was visualized. The averaged mass density estimated by integrating the distribution map was in close agreement with estimation from dynamic measurements and between repeating experiments. The capability of this novel technique to sense, quantify and visualize the mass distribution of nTiO2 provides a valuable approach to investigate the behavior of nanoparticles at the interface of flow and rough surfaces.