Session: 10.2.3 - Interfacial Phenomena and Flows III

Paper Number: 158209

158209 - Drop Splashing With Complex Fluids 

Abstract: 

Particle-laden drop impact is of practical importance to a variety of commercial applications including additive manufacturing and coating technologies. The particles impart significant changes to the droplet rheology at high concentrations and at dilute concentrations, the particles do not influence the bulk properties. However, although the particles do not influence the bulk properties, recent studies have shown that suspension droplets splash at lower Weber numbers than the pure fluid counterparts with no existing theories to describe the mechanism. Previous theories on the relevant mechanism for splashing have included nearly every classic fluid dynamic instabilities including Rayleigh-Taylor, Rayleigh-Plateau, and the Kelvin Helmholtz theory - which has been debated alongside the role of lift forces near the lamella as the source of the splashing. To date, the exact mechanism for droplet splashing seems to be elusive partly due to the fact that the dominant source of the splash depends on a variety of factors including surface tension, rheology (e.g..,shear and extensional viscosity, shear thinning, etc.), density,  drop size, velocity,  multicomponents, substrate wettability and roughness, and ambient conditions (e.g., pressure, temperature, etc.). In this study, we report a previously unexplored gap in the understanding of particle-laden drop splashing by exploring the role of small density contrasts between the bulk fluid and the particle. The physics of the drop impact was explored using a custom-built drop impact setup with the release height controlled by a linear stepper motor and the droplet generated out of a needle and produced by a syringe pump. The dynamics was captured with a high-speed camera from the side and bottom views on parafilm-mounted glass and Glaco-coated glass substrates. The droplets were made of ultrapure water and laden with various types of colloidal particles including glass, polyethylene, polystyrene, polyethylene glycol, and various clays (e.g., laponite, kaolinte and bentonite). The concentrations were controlled between the dilute to semi-dilute concentrations using a roller mixer and the rheological properties verified with a rheometer and the dripping-onto-substrate method for the uniaxial extensional flow characterization. We use the term particle loosely to mean any colloid or hydrocolloid with insignificant extensional effects resisting contactline movement. We find that provided that there is a small density contrast between the bulk fluid and the particle, the critical Weber number for splashing is diminished in even for dilute suspensions. The suggested mechanism at play is due to the destabilizing role of density heterogeneities in the lamella and not due to other scenarios such as earlier contact times of the particle-laden droplets when compared with the pure-liquid case. 

Presenting Author: Min Pack Baylor University

Presenting Author Biography: