Integrating in-situ Shear Rheology with Neutron Reflectometry for Structural and Dynamic Analysis of Interfacial Systems

Abstract

The study of the structure and mechanical properties of complex fluid interfaces has gained increasing interest in recent decades as a result of its significant scientific relevance to the understanding of biological systems, drug development, and industrial applications. The in situ combination of molecular-level structural measurements with the assessment of dynamical (rheological) properties is particularly valuable, as comparing measurements conducted on separate samples under challenging-to-reproduce experimental conditions can be problematic. In this work, we present a new sample environment at the FIGARO instrument, the horizontal neutron reflectometer at the Institut Laue-Langevin, which includes an interfacial shear rheometer operating in the Double Wall-Ring (DWR) geometry, compatible with commercial rotational rheometers. This innovative setup enables simultaneous structural (neutron reflectometry) and dynamical (shear interfacial rheology) measurements on the same sample.

Figures (7)

• Figure 1: Sketch of the DWR on FIGARO setup. (a) Perspective of the whole ensemble attached to the rheometer frame including the support table. (Inset: Detail of the DWR ensemble). (b) Top view with a half-height cutting plane showing the disposition of the interfacial rheology measurement system, incident neutron beam and footprint, interfacial pressure balance position, and barrier travel range.
• Figure 2: Sketch of the DWR cross-sectional geometry. Only the right half is shown, taking advantage of the rotational symmetry to highlight the details. The different radii are labelled as in reference Sanchez-Puga2024.
• Figure 3: Sketch of the electrical connections. The DAQ board, used to acquire the raw analogue signals from the rheometer, and the control units of the rheometer and Langmuir trough are all connected via USB to a PC running custom-made acquisition/analysis software, rheometer control software, and Langmuir trough control software. The interfacial pressure sensor and the barrier motor of the Langmuir trough are connected to their respective control units via serial connections. The four analogue signals from the rheometer are connected to the DAQ board via the analogue input channels using single-ended connections, with all channels sharing the same ground reference.
• Figure 4: Panel A: $|AR^*(f)|$ at $\gamma = 3$ %. Panel B: $|AR^*(\gamma_s)|$ at $f = 0.5$ Hz. Panel C: $\varphi(f)$ at $\gamma = 3$ %. Panel D: $\varphi(\gamma_s)$ at $f = 0.5$ Hz. DPPC monolayers on ACMW (circles) and D$_2$O (squares) subphases, at $\Pi = 25$ mN/m (red), $\Pi = 35$ mN/m (green), and $\Pi = 45$ mN/m (blue). Clean air/water interface is represented with purple triangles. A dashed line with slope $2$ in panel A, a dashed horizontal line at $10^{-3}$ in panel C and a dashed horizontal line at $\pi$ in panels B and D have been plotted to guide the eye.
• Figure 5: Panel A: Loss modulus, $G_s^{\prime\prime}(f)$ at $\gamma_s = 3$ %. Panel B: Loss modulus, $G_s^{\prime\prime}(\gamma_s)$ at $f = 0.5$ Hz. Panel C: Storage modulus, $G_s^{\prime}(f)$ at $\gamma_s = 3$ %. Panel D: Storage modulus, $G_s^{\prime}(\gamma_s)$ at $f = 0.5$ Hz. DPPC monolayers onto ACMW (circles) and D$_2$O (squares) subphases, at $\Pi = 25$ mN/m (red), $\Pi = 35$ mN/m (green), and $\Pi = 45$ mN/m (blue). The dotted line indicates the inertia-limited sensitivity.