Eur. Phys. J. E (2022) 45: 43
https://doi.org/10.1140/epje/s10189-022-00200-7

Regular Article - Soft Matter

Effect of an excess of surfactant on thermophoresis, mass diffusion and viscosity in an oily surfactant-stabilized ferrofluid

¹ Institute of Physics, University of Latvia, Miera 32, LV-2169, Salaspils, Latvia
² CNRS - Lab. PHysico-chimie des Electrolytes et Nanosystèmes InterfaciauX (PHENIX), Sorbonne Université, Case 51-4 Place Jussieu, 75005, Paris, France
³ Dpt de Physique, Univ. Cergy-Pontoise, 33 Bd du port, 95011, Cergy-Pontoise, France

^a viesturs.sints@lu.lv

Received: 24 September 2021
Accepted: 11 April 2022
Published online: 5 May 2022

Abstract

The effect of an excess of surfactant on the thermophoresis of a sterically stabilized ferrofluid is investigated experimentally by forced Rayleigh scattering (FRS). The experiments are performed with a stable magnetic fluid sample to which controlled amounts of surfactant are added. A decrease in the thermally induced transport of magnetic nanoparticles is observed while increasing the temperature T. The positive Soret coefficient $S_{\mathrm{T}}$ decreases by adding 2 vol% of surfactant at room temperature. As shown by FRS relaxation, this decreasing is mainly associated with a reduction of the interaction between the carrier fluid and individual nanoparticles. No significant effect of extra surfactant on the sign of $S_{\mathrm{T}}$ is observed at higher T’s (up to $\sim 85{}^{\circ }$C). Dynamic light scattering at room temperature reveals the presence of a small amount of clusters/aggregates in the samples, which are hardly detectable by FRS relaxation. The presence of these small clusters/aggregates is confirmed by a rheological probing of the fluid properties. Whatever T, a small amount of added surfactant first causes a decrease of the ferrofluid viscosity, associated with a 10% decreasing of the flow activation energy. Further on, viscosity and activation energy both recover at higher excess surfactant concentrations. These results are analyzed in terms of saturation of the surfactant layer, concentration of free surfactant chains and heat of transport of the nanoparticles.