Stress relaxation in quasi-two-dimensional self-assembled nanoparticle monolayers

Abstract

We experimentally probed the stress relaxation of a monolayer of iron oxide nanoparticles at the water-air interface. Upon drop-casting onto a water surface, the nanoparticles self-assembled into islands of two-dimensional hexagonally close packed crystalline domains surrounded by large voids. When compressed laterally, the voids gradually disappeared as the surface pressure increased. After the compression was stopped, the surface pressure (as measured by a Wilhelmy plate) evolved as a function of the film aging time with three distinct timescales. These aging dynamics were intrinsic to the stressed state built up during the non-equilibrium compression of the film. Utilizing x-ray photon correlation spectroscopy, we measured the characteristic relaxation time (τ) of in-plane nanoparticle motion as a function of the aging time through both second-order and two-time autocorrelation analysis. Compressed and stretched exponential fitting of the intermediate scattering function yielded exponents (β) indicating different relaxation mechanisms of the films under different compression stresses. For a monolayer compressed to a lower surface pressure (between 20 mN/m and 30 mN/m), the relaxation time (τ) decreased continuously as a function of the aging time, as did the fitted exponent, which transitioned from being compressed (>1) to stretched (<1), indicating that the monolayer underwent a stress release through crystalline domain reorganization. However, for a monolayer compressed to a higher surface pressure (around 40 mN/m), the relaxation time increased continuously and the compressed exponent varied very little from a value of 1.6, suggesting that the system may have been highly stressed and jammed. Despite the interesting stress relaxation signatures seen in these samples, the structural ordering of the monolayer remained the same over the sample lifetime, as revealed by grazing incidence x-ray diffraction.

Stress breaks universal aging behavior in a metallic glass

“…Figure 5 also reveals abrupt variations in the contrast at the highest stress. Such intermittent contrast variations have been seen during martensitic phase transformations in crystalline alloys 14 , local but collective rearrangements in 2D nanoparticle monolayers 32 , as well as in MGs during cooling 17 , and are referred to as structural avalanches. Seen at the highest applied stress, the intermittent dynamics may thus indicate that the system is close to global yielding, i.e., shear banding 33,34 .…”

From Femtoseconds to Hours—Measuring Dynamics over 18 Orders of Magnitude with Coherent X-rays

“…Boucheron et al probed the stress-relaxation of a monolayer of nanoparticles on the water surface [116]. The particles formed island that have been laterally compressed.…”

Unraveling the Role of Excess Ligand in Nanoparticle Pattern Formation from an Evaporatively Dewetting Nanofluid Droplet

“…We postulate that, due to the inaccessibility of the excess ligand into the excluded volume of NP assembly (supported from OPM analysis), a depletion force is acting between the NPs, which helps the formation of macroscopically continuous sheet. 34−36 However, such kind of surface compression occasionally results in the formation of domain boundaries, point defects, and voids, 37,38 therefore losing the long-range ordering within the monolayer (see SI Section SV, Figure SV-1). With further increase in V ex thiol /V NP up to 24 (Figure 2d−f), these branches grow in size with concomitant development of fingering and nucleation of holes within the monolayer.…”