Linear reciprocating tribometer for in situ neutron reflectometry of soft matter

Neutron reflectometry is a technique for measuring structure near planar
interfaces that has been previously used to non-destructively characterize
the polymer density of hydrated, dilute, and soft materials. Previous
investigations have conducted neutron reflectometry measurements of
liquids, gels, emulsion, and polymer solutions at rest, in compression,
and subject to shear stress. However, correlating structure with
tribological properties of soft materials presents significant
experimental challenges for prior instruments due to wall slip, sample
thickness, and structural heterogeneity (eg., depth-wise gradients). A
linear reciprocating tribometer offers several advantages for in situ
neutron reflectometry studies, including: uniform velocity profiles,
constant shear stress over large regions of interest, and independent
control of normal force and sliding velocity during measurements. This
work outlines basic considerations for the design of a custom linear
reciprocating tribometer that operates in a neutron beamline and includes
commissioning measurements. The tribometer is designed to compress soft
and hydrated materials against linearly reciprocating silicon disks. The
three key design considerations for this tribometer are (1) safety, (2)
neutron transmission, and (3) sample positioning. This instrument design
will enable in situ studies of soft matter and illuminate the role of
interfacial structure on tribological phenomena.