arXiv subjects

Hayden Robertson

Publications and source records attributed to Hayden Robertson.

3 recordsLinked to original sources

arXiv2026

A Multi-scale Investigation of Aqueous Foams Stabilised by PNIPAM Microgels

Aqueous foams possess multiple structural motifs across different length scales: macroscopic foam, bubbles, foam films and the air/water interface. In this study, macroscopic foams are generated by sparging gas through an aqueous dispersion of PNIPAM microgels which act as foam stabilisers due to their surface activity. The stiffness of the microgels and thus their interfacial activity are tuned by variation of the cross-linker density. The effect of the cross-linker density and the microgel concentration on the resulting foam formation properties (foamability) and the foam stability are investigated. A lower cross-linker density and a higher microgel concentration enhance the foamability, generate foams with smaller bubbles and higher liquid fractions, and increase the foam stability. These observations are correlated with the microgel behavior at the single air/water interface examined by pendant drop tensiometry and Langmuir compression experiments as well as the mobility in single free-standing foam films determined using a Thin Film Pressure Balance. Our findings highlight good agreement across all length scales: increased foamability correlates with a faster decrease in surface tension, and higher foam stability with a higher surface elastic modulus of a microgel-covered single air/water interface and decreasing mobility in foam films.

cond-mat.soft↗

arXiv2026

Anomalously Large Screening Lengths in Highly Concentrated Electrolytes: A Matter of Experimental Conditions

Considerable debate exists about anomalous underscreening. Whilst surface force apparatus (SFA) measurements have confirmed anomalously large screening lengths ($\lambda$) in concentrated aqueous electrolytes, colloidal probe atomic force microscopy (CP-AFM) measurements have not. We demonstrate that by tuning experimental conditions, similarly large $\lambda$ can be achieved in concentrated aqueous LaCl$_3$ electrolytes with CP-AFM. Structural forces were also observed at high LaCl$_3$ concentrations, which we suggest stem from the formation of diffuse ion clusters, and that the remaining counter-ions yield the large deduced $\lambda$.

cond-mat.soft↗

arXiv2025

In situ vs ex situ: Comparing the structure of PNIPAM microgels at the air/water and air/solid interfaces

For studying the structure of microgel particles at the air/water interface, specular and off-specular X-ray reflectivity (OSR/XRR) allows in situ measurements without any labelling techniques. Herein we investigate the vertical and lateral structure of poly(N-isopropylacrylamide) (PNIPAM) microgels (MGs) at the air/water interface and the effect of Langmuir-Blodgett (LB) transfer onto solid substrates. The initial ex situ atomic force microscopy (AFM) scans of LB-transferred MGs at the air/solid interface reveal strong lateral 2D hexagonal ordering across a broad range of lateral surface pressures at the air/water interface before LB-transfer. Notably, for the first time, these results were confirmed by OSR, demonstrating the existence of the long-range hexagonal ordering at low and intermediate surface pressures. For in situ conditions and upon uniaxial compression at the air/water interface, the MG lattice constant decreases non-monotonically. This indicates the formation of domains at low pressures that approach each other and only compress when the surface isotherm reaches a plateau. Comparing the results of in situ and ex situ measurements, our study demonstrates a clear transfer effect during the LB-deposition on the lateral ordering of the MGs: the distance between the particles decreased during LB-transfer, and at high pressures ($\Pi\,>\,17\,\mathrm{mNm^{-1}}$) a second distance occurs indicating small domains with hexagonal internal ordering. The novel surface characterisation approaches debuted here highlight the use of both XRR and OSR to probe the vertical and lateral structure of adsorbed MGs, offering in situ, non-invasive insights without the need for doping or transfer-induced artefacts.

cond-mat.soft↗