# Study of liquid surfaces and interfaces using neutron reflection

#### Abstract

In this dissertation we describe the thermal neutron reflectometer recently installed at the Rhode Island Nuclear Science Center (RINSC) and present its application on a study of liquid surfaces and interfaces. The reflectometer utilizes a "slab-like" beam with an adjustable angle of incidence on the horizontal sample surface. It permits reflection studies with the neutron beam slanting upward or downward in the range up to 0.03 radian allowing measurements to be made from above or from below (e.g., through the sample substrate). Using a disk chopper a time-of-flight resolution of $\approx$5% ($\Delta$Q$\sb{\rm z} \approx$ 0.001 A$\sp{-1}$) is achieved for a flight path of 4.5 m and a wavelength of 1.8 A. This instrument covers a Q$\sb{\rm z}$-range from $\approx$0.01 to 0.1 A$\sp{-1}$ and measures reflectivities down to about 10$\sp{-4}$. We present the results of measurements of specular reflectivity for a two-layer oil-water-surfactant system where the oil floats on top of the water. From the reflectivity data we deduce the surface and interface roughness for the air-oil and the oil-water interfaces of the system using a Debye-Waller-like factor. The roughnesses obtained are consistent with the predicted values from an extended capillary wave model, which we develop here for the thermally excited surface and interface roughness of a binary-liquid system. We also discuss the "slip" at the liquid-liquid interface due to the neglect of fluid viscosity in the model. For this purpose, the linearized Navier-Stokes equation is used to analyze the viscosity damping effect on capillary waves. ^

#### Subject Area

Physics, Optics|Physics, Fluid and Plasma

#### Recommended Citation

U-Ser Jeng, "Study of liquid surfaces and interfaces using neutron reflection" (1996). Dissertations and Master's Theses (Campus Access). Paper AAI9723561.
http://digitalcommons.uri.edu/dissertations/AAI9723561

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