An official website of the United States government
Hypothesis The interaction of active particles with walls can explain discrepancies between experiments and theory derived for particles in the bulk. For an electric field driven metallodielectric Janus particle (JP) adjacent to an electrode, interaction between the asymmetric particle and the partially screened electrode yields a net electrostatic force – termed self-dielectrophoresis (sDEP) - that competes with induced-charge electrophoresis (ICEP) to reverse particle direction. Experiments The potential contribution of hydrodynamic flow to the reversal is evaluated by visualizing flow around a translating particle via micro-particle image velocimetry and chemically suppressing ICEP with poly(l-lysine)-g-poly(ethylene glycol) (PLL-PEG). Mobility of Polystyrene-Gold JPs is measured in KCl electrolytes of varying concentration and with a capacitive SiO2 coating at the metallic JP surface or electrode. Results are compared with theory and numerical simulations accounting for electrode screening. Findings PLL-PEG predominantly suppresses low-frequency mobility where propulsive electro-hydrodynamic jetting is observed; supporting the hypothesis of an electrostatic driving force at high frequencies. Simulations and theory show the magnitude, direction and frequency dispersion of JP mobility are obtained by superposition of ICEP and sDEP using the JP height and capacitance as fitting parameters. Wall proximity enhances ICEP and sDEP and manifests a secondary ICEP charge relaxation time dominating in the contact limit.
more »
« less
Effects of tunable hydrophobicity on the collective hydrodynamics of Janus particles under flows
Active colloidal systems with nonequilibrium self-organization constitute a long- standing, challenging area in material sciences and biology. To understand how hydrodynamic flow may be used to actively control self-assembly of Janus particles (JPs), we developed a model for the many-body hydrodynamics of amphiphilic JPs suspended in a viscous fluid with imposed far-field background flows [Fu et al., J. Fluid Mech. 941, A41 (2022)]. In this paper we alter the hydrophobic distribution on the JP-solvent interface to investigate the hydrodynamics that underlies the various morphologies and rheological properties of the JP assembly in the suspension. We find that JPs assemble into unilamellar, multilamellar, and striated structures. To introduce dynamics, we include a planar linear shear flow and a steady Taylor-Green mixing flow and measure the collective dynamics of JP particles in terms of their (a) free energy from the hydrophobic interactions between the JPs, (b) order parameter for the ordering of JPs in terms of alignment of their directors, and (c) strain parameter that captures the deformation in the assembly. We characterize the effective material properties of the JP structures and find that the unilamellar structure increases orientation order under shear flow, the multilamellar structure behaves as a shear thinning fluid, and the striated structure possesses a yield stress. These numerical results provide insights into dynamic control of nonequilibrium active biological systems with similar self-organization.
more »
« less
- Award ID(s):
- 1951600
- PAR ID:
- 10471444
- Editor(s):
- Howard A. Stone
- Publisher / Repository:
- American Physical Society
- Date Published:
- Journal Name:
- Physical review fluids
- Volume:
- 8
- Issue:
- 5
- ISSN:
- 2469-990X
- Page Range / eLocation ID:
- 050501
- Subject(s) / Keyword(s):
- collective behaviour membranes boundary integral methods
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Studying the behavior of anisotropic particles at fluid interfaces is a rapidly expanding field, as understanding how the introduced anisotropy affects the resulting properties is essential in the engineering of interfacial systems. Surface anisotropic particles, also known as Janus particles (JPs), offer new possibilities for novel applications due to their amphiphilicity and stronger binding to fluid interfaces compared to homogeneous particles. Introducing surface anisotropy creates complexity as the orientation of interfacially bound particles affects interparticle interactions, a contributing factor to the microstructure formation. In this work, we have investigated the microstructure of JP monolayers formed at the air–water interface using particles with different degrees of amphiphilicity and examined the response of the networks to applied compressions. Our findings demonstrate that JPs amphiphilicity is a crucial factor governing their orientation at the interface, which in turn dictates the complexity of the capillary interactions present and the mechanical properties of the ensuing networks.more » « less
-
Abstract Understanding the interactive behavior of Janus particles (JPs) is a growing field of research. The enhancement in binding energy, in comparison to homogenous particles, and the dual characteristic of JPs open up new possibilities for novel applications. In many such applications, interfacial materials become subjected to flows that produce dilational and shear stresses. Therefore, it is important to understand the impact that the Janus character brings to interfaces. In this work, we study the microstructure of two-dimensional (2D) JP monolayers formed at the air–water interface and examine the shear viscoelasticity with an interface rheometer that was adapted for in situ surface pressure control via a Langmuir trough. We extend concepts from bulk rheology to data obtained from interfacial rheology as a tool to understand and predict the monolayer’s viscoelastic behavior. Finally, by calculating the time relaxation spectrum from the measured 2D dynamic moduli, we conclude that a phenomenon similar to glass transition is taking place by analogy.more » « less
-
Complex fluid interfaces are commonplace in natural and engineered systems and a major topic in the fields of rheology and soft matter physics, providing boundary conditions for a system’s hydrodynamics. The relationship between structure and function dictates how constituents within complex fluids govern flow behavior via constituents changing conformation in response to the local microenvironment to minimize free energy. Both hydrodynamics, such as shear flow, and the presence of air–liquid interfaces are principal aspects of a complex fluid’s environment. The study of fluid interfaces coupled to bulk flows can be uniquely advanced through experimentation in microgravity, where surface tension containment can be achieved at relatively large length scales. This computational investigation assesses flow in the ring-sheared drop (RSD), a containerless biochemical reactor operating aboard the International Space Station for the study of complex fluids and soft matter physics. Specifically, the hydrodynamic effects of a generalized Boussinesq–Scriven interface with a shear-thinning surface shear viscosity are examined in flow regimes where the air–liquid interface remains coupled to the Newtonian bulk fluid. The results verify this interfacial model’s ability to affect system-wide hydrodynamics under specific parameter regimes, enabling future model validation with high-precision rheological measurements.more » « less
-
Abstract The engineering problems involving clay under non‐isothermal conditions (e.g., geothermal energy harvest, landfill cover system, and nuclear waste disposal) are multiscale and multiphysics by nature. The nanoscale hydrodynamics of clay at elevated temperature is essential in developing a physics‐based multiscale model for clay under non‐isothermal conditions. The nonequilibrium molecular dynamics (NEMD) is a useful tool to study the nanoscale hydrodyndamics of clay. This article presents an NEMD modeling of hydrodynamics of clay nanopores at elevated temperatures. Water flow confined in pyrophyllite and montmorillonite clay nanopores is investigated. The nonequilibrium state is maintained by uniformly exerting an external force on each water molecule. The NEMD simulations have provided a molecular‐scale perspective of temperature effect on clay‐water density, water flow velocity, shear viscosity, clay‐water slip length, hydraulic conductivity, and clay‐water friction coefficient. The numerical results have shown a strong temperature dependence of fluid flow velocity, shear viscosity, clay‐water slip length, and hydraulic conductivity at the nanoscale. We have validated the applicability of cubic law in determining hydraulic conductivity at the nanopore scale at elevated temperatures. It is found from our numerical results that slip clay‐water boundary condition is an essential factor in properly determining nanoscale fluid flow velocity. By numerical examples, we also study the impact of nanopore size and clay layer thickness on the hydrodynamics of the clay‐water system.more » « less
