Effect of Ultrasound Standing Wave-Induced Acoustophoresis in Monoglyceride Oleogel Structuration

Ultrasound standing waves (USW) produce a force capable of displacing micrometer-sized free-flowing particles in a fluid, wherein this phenomenon is also referred to as acoustophoresis. However, the effect of acoustophoresis on dynamically changing and growing crystal networks is unclear. An example of such a system are monoglyceride (MG)-based oleogels, which are free-flowing lipids (e.g., vegetable oils) structured with a lipid-crystal network. In this work, we use MG oleogels as an example system to investigate the acoustophoretic effect on the structuration of a growing crystal network. For this purpose, multifaceted characterization is conducted utilizing optical and coded excitation scanning acoustic microscopy as well as small-angle X-ray scattering, respectively. The optical microscopy results show that USW produces local density differences of the structuring crystalline material and induces the orientation of the MG platelets. X-ray diffraction measurements confirm these findings and show a 23% average increase in MG platelet correlation length, which can be linked to platelet thickness, as well as an increase in the MG nanoplatelet surface smoothness. These findings produce a foundation for better understanding the effect of acoustophoresis in dynamically developing lipid-based materials and illuminate the mechanical changes that arise because of USW treatment.