Article pubs.acs.org/Langmuir
Spontaneous Particle Transport through a Triple-Fluid Phase Boundary Bum Jun Park†,‡ and Daeyeon Lee*,† †
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Department of Chemical Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, South Korea
‡
S Supporting Information *
ABSTRACT: We investigate the spontaneous transport of a single particle through an air− water−oil triple phase boundary that is formed by placing a thin oil lens at an air−water interface. We find two distinct transition regimes: a particle initially accelerates upon its adsorption to the air−water−oil triple phase boundary from the air−water interface; subsequently, the particle decelerates after spontaneously detaching from the triple phase boundary. In the first stage, which we call the capillarity regime, the difference in the particle attachment energy to the three fluid−fluid interfaces accounts for the observed initial acceleration. Once it detaches from the air−oil interface and resides solely at the oil−water interface, the particle decelerates due to viscous drag; hence, we call this phase the relaxation regime. We show that the shape of oil lens as well as the size of the particles has a significant influence on the dynamics of particle transport through the triple-fluid phase boundary.
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INTRODUCTION Particles spontaneously and irreversibly attach to interfaces between two immiscible fluids to lower the surface free energy.1,2 This tendency of particles to attach to fluid−fluid interfaces enables the stabilization of multiphasic fluid mixtures such as emulsions and foams and also allows for the generation of semipermeable vesicles such as colloidosomes.3 The adsorption behavior of a particle to a fluid−fluid interface depends on its size and shape as well as the properties of each fluid, such as the surface tension and the curvature of the fluid interface.4−9 Thus far, most studies in this area have focused on the interactions of particles with interfaces between two immiscible fluids.10−15 The behavior of particles near or at triple phase boundaries, however, has not been extensively studied; only a limited number of studies have reported on the interactions between particles and a three-fluid phase boundary that would form, for example, between air, water, and oil.16 Such investigation is potentially important in understanding the efficacy of oil adsorbent particles in the remediation of oil spills or the interaction of soil or minerals with complex mixtures of air, oil, and water in enhanced oil recovery.17,18 In these situations, particles would inevitably come in contact with an air−water− oil triple phase boundary.19 In this article, we report on the behavior of the spontaneous transport of single spherical particles, initially residing at an air−water interface, through an air−water−oil triple-fluid phase boundary. We examine the effect of particle size, different oils, and oil volume on the transition behavior. The particle transition through the triple phase boundary exhibits two distinct regimes: acceleration and deceleration, the origin of which is discussed. © 2013 American Chemical Society
MATERIALS AND METHODS
An air−water−oil (A/W/O) triple-fluid phase boundary is generated by gently placing a small volume of oil on a neat water surface. The triple phase boundary is the intersection of the air−water (A/W), air− oil (A/O), and oil−water (O/W) interfaces present in the system. As a reference system, we use octane to form the oil lens. The air−water interface is slightly deflected downward around the triple phase boundary due to the weight of the oil lens.20 A polystyrene particle (PS) with diameter, 2R = 200 μm, prepared by using microfluidics (see the detailed method in Figure S1 of the Supporting Information), is directly placed at the air−water interface using a micropipet. The size of the particle is chosen such that we can monitor the particle using optical microscopy and, at the same time, can neglect the deformation of the interfaces due to the effect of gravity because of a sufficiently small Bond number (Bo ≈ 10−3), which is the ratio of body forces (i.e., gravity and buoyancy) to the surface tension force. The sample cell is immediately sealed by placing a coverslip on top of the sample chamber and sealing it with vacuum grease to minimize evaporation and convection. The particle initially at the curved air−water interface spontaneously approaches the triple phase boundary due to gravity and eventually transfers to the oil−water interface passing through the triple phase boundary (see the example movie, MovieS1, in the Supporting Information). A high-speed camera with frame rates in the range of 300−500 frames/s is used to capture the motion of the particle, and its trajectory is analyzed by using the ImageJ software.21
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RESULTS AND DISCUSSION Polystyrene (PS) particles initially confined at the air−water interface spontaneously adsorb to the triple phase boundary and subsequently translate toward the center of the oil (i.e., Received: March 30, 2013 Revised: July 15, 2013 Published: July 15, 2013 9662
dx.doi.org/10.1021/la401183u | Langmuir 2013, 29, 9662−9667
Langmuir
Article
Figure 1. Spontaneous transport of a single polystyrene (PS) particle with 2R = 200 μm from the air−water interface to the octane−water interface through the air−water−oil triple phase boundary. The volume of octane is 90 μL. (a) Overlaid microscopy image stack showing the particle motion for 0.38 s. The scale bar is 1 mm. The red arrow indicates the direction of particle motion. Inset is the particle prior to the adsorption and transition through the triple phase boundary. (b) Particle position upon the transition through attachment energy calculation. The red line indicates the center of mass of the particle. (c) Attachment energy profile and the corresponding force as a function of the radial distance, x. The schematic on the top indicates calculated particle positions around the triple phase boundary. (d) Velocity profiles from experiments and theoretical models. Inset is the migration distance, d, of the particle as a function of time.
fluid interface displaced by the particle. Since the curvature of an oil lens is significantly smaller than that of the particle, the effect of Laplace pressure on ΔEatt(x,z) is negligible. Note that eq 1 does not contain a term with the three-phase contact angle of the particles at the air−oil interface (θAO). The derivation of eq 1 provided in the Supporting Information shows that this term is not necessary to determine the particle attachment energy (ΔEatt(x,z)). The three-phase contact angles of the polystyrene particles at the oil−water and air−water interfaces are θOW = 105 ± 1° and θAW = 73 ± 2°, respectively. The contact angle of an oil droplet on a PVA-modified polystyrene (θAO) surface is
