Nanofluidic Cells Imaging and Scattering Measurements of Liquids and Gases

A. Kanwal, E.H. Gann, B.R. Ilic, G. Holland, S. Mukherjee, D. DeLongchamp, J.A. Liddle
National Institute of Standards and Technology,
United States

Keywords: polarized resonant soft X-ray scattering, nanofludics

Summary:

Synchrotron x-ray measurements can provide data on chemistry, chemical bonding, and molecular orientation. In principle, they are ideally suited to the investigation of dynamic processes at solid-liquid interfaces. Recently, Polarized Resonant Soft X-ray Scattering (PRSoXS) has been used to provide information on the molecular orientation in solid films. [1, 2] While useful for solid films, this powerful technique has largely been inapplicable to biological molecules, structural nanocomposites, and liquid crystals. Such samples typically require liquid or gas environments which are incompatible with the vacuum needed for soft X-rays. To study these important materials, it is necessary to develop fluidic cells to protect the sample from the vacuum and vice versa. Commercial devices comprise two chips sealed together with silicon nitride membranes forming a closed liquid cell. However, the relatively large (≈ 100 μm) membranes either bulge out, resulting in a thick liquid layer that causes too much absorption, or they collapse, resulting in insufficient scattering. In both cases, the volume of liquid being interrogated is unknown, precluding quantitative measurements. A robust solution is therefore needed to enable PRSoXS for liquids and gases. We have developed a monolithic, scalable fabrication process that yields a single chip (Figure 1a) with two nitride membranes supported with pillars. Initial tests with membrane thickness of 50 nm with regularly spaced pillar supports result in membrane deflections in the range of 50 nm to 100nm for pressures ranging from 20 MPa to 100 MPa [3]. Unfortunately, the regular spacing of the pillars results in a strong scattering signal which overwhelms the signal of interest from the sample (Figure 1b). However, by independently randomizing the pillar spacing, shape, and orientation, the resulting diffraction pattern becomes that of an amorphous material, comprising a few diffuse concentric rings as shown in (Figure 1c) for a single membrane. This design will be used for real complex fluid samples. The monolithic pillar-supported fluidic cell not only makes scattering data collection possible, but also simplifies the data analysis. Finally, we note that these nanofluidic devices are also fully compatible with the transmission electron microscope, enabling multimodal measurement comparisons.