Water-in-oil emulsion drop-wall impact: Effect of viscosity and water particles on slip length over an oleophilic surface

Deformation of biological cells, tissues, and similar soft materials is often considered linearly elastic; however, the assumption is only valid in a very limited stress range and often leads to significant errors in mechanical evaluation. We demonstrated the hyper-elastic behavior of ultra-soft poly(N-isopropyl acrylamide) (PNIPAm) microgels (USPNMs) in a converging channel flow, as a representation for biological tissues. The hyper-elasticity of USPNMs in response to a broad range of deformation was characterized at the centerline of the converging flow. We introduced a carrier fluid consisting of baby hydrogels (avg. diameter, 10 μm) and oil that carried the hydrophilic USPNM sample (avg. diameter, 100 μm) on the centerline of oil background fluid. By “baby hydrogel,” we mean small PNIPAm particles obtained during USPNM synthesis, using which, enabled settling-free flow, prevented wall contact, and enhanced carrier fluid viscosity for increased stresses at lower flowrates. Furthermore, drastic reduction of interfacial tension was observed in the converging area due to contact of baby gels with USPNM particles in the carrier fluid. The shear and elongational stresses were balanced with the elastic stress and interfacial Laplace pressure. As a result, we obtained a stress–strain curve from the microscopic images during flow. The non-linear stress–strain curve was characterized by conventional hyper-elastic models. The elastic modulus of the synthesized USPNM was 24 Pa, which is as low as animal brain tissue. This method holds great potential for implementing in similar hyper-elastic systems, enabling accurate mechanical evaluations in the field of soft materials, biology, and medicine.

Hyper-elastic behavior of soft-tissue like microgels in two-phase converging microchannel flow

Hussain,

Rahmannezhad,

Choi

et al. 2023

Numerical investigation of the natural transition in boundary layers on underwater axisymmetric bodies with superhydrophobic surfaces

Liu,

Zhang

2024

The natural transition in boundary layers on axisymmetric bodies with superhydrophobic surfaces is studied using numerical methods. By boundary layer, we mean the boundary layer around the forebody and around the parallel body of an underwater vehicle. A method of calculating basic laminar flows on axisymmetric bodies with superhydrophobic curved surfaces is proposed, and a method of linear stability analysis for the boundary layers that considers the slip-velocity boundary condition for small disturbances is established. The eN method is used to predict the transition locations of the boundary layers. On a superhydrophobic surface, the slip velocity on the wall increases and then decreases along the streamwise direction, resulting in a maximum value at the forebody. The boundary layer thickness on a superhydrophobic surface becomes smaller than that on an ordinary surface. The superhydrophobic surface stabilizes the boundary layer and thus delays the natural transition. As the slip length on the superhydrophobic surface increases, the unstable zone shrinks, and the transition location moves further downstream, indicating that the delay effect of the superhydrophobic surfaces becomes stronger. As the oncoming flow velocity increases, the transition location on the superhydrophobic surface moves upstream and then downstream, leading to a “dangerous” velocity, at which the transition location is closest to the leading edge. The underlying mechanism of the dangerous velocity phenomenon is discussed: An increasing velocity has both stabilizing effect and destabilizing effect on the boundary layer.

Droplet impact outcomes of emulsions on smooth and microstructured surfaces

Quetzeri-Santiago,

Oropeza-Ramos,

Sánchez-Arévalo

2024

This study explores the impact dynamics of emulsion droplets, with a dispersed phase of either silicone oil, toluene, or heptane, and water as the continuous phase, on both smooth and microstructured surfaces fabricated via photolithography. By preparing emulsions without surfactants, we isolated the effects of surface morphology and liquid properties on droplet behavior. We characterized the rheology of the emulsions and their droplet size distributions. The impact dynamics were recorded using a high-speed camera in a shadowgraph configuration, with analysis performed through image processing techniques. Our results indicate that at higher impact velocities, water exhibits the largest spreading diameter (dmax) on smooth surfaces, while emulsions with higher dispersed phase concentrations show reduced spreading due to increased energy dissipation. On microstructured surfaces, denser structures enhance resistance to spreading and trigger complex phenomena such as Worthington jets and secondary droplets, which are not observed on smooth surfaces. Additionally, we observed a transition in bouncing behavior for the silicone oil 50 cSt 20 v/v% emulsion on Glaco-coated surfaces, attributed to the infiltration of silicone oil into the Glaco microstructure, creating a suction force that prevents bouncing. These findings offer valuable insights for optimizing industrial processes like inkjet printing and pesticide application, where precise control of droplet behavior is crucial.