Trajectory and Transit Patterns of Isolated Nanoparticles in Structured Micromodels

Research Article

Austin J Chem Eng. 2014;1(2): 1006.

Trajectory and Transit Patterns of Isolated Nanoparticles in Structured Micromodels

Sajjadiani S1, Javadpour F2 and Jeje AA1*

1Department of Chemical and Petroleum Engineering, University of Calgary, Canada

2Department of Geosciences, University of Texas, USA

*Corresponding author: Jeje AA, Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada

Received: May 30, 2014; Accepted: July 09, 2014; Published: July 14, 2014


Transport of suspensions of ultrafine and colloidal particles through porous structures is a common occurrence. Often, of interest is how far the particles would penetrate into the structure on advective streams. Experiments were conducted to record the paths of nano particles in dilute suspensions through micro models of porous media. The particles are primarily propelled by hydrodynamic forces through regular arrays of cylindrical posts arranged between two closely-spaced flat surfaces. The setup consisted of the test cell, an inverted microscope, a high speed camera, a data processor, a precise actuating syringe pump, and spherical silica particles (0.2, 0.5 and 1.5 μm diameter) that encapsulated a fluorescent dye. The particle trajectories deviated from the streamlines for particle-free fluids through the domain, and the slip condition was prevalent in an environment in which the fluid velocity field and the particle motion affected each other. The trajectories show evidence of Brownian motion more significantly for the smaller particles that were also dispersed faster through the less porous media.

Keywords: Particle trajectory; Porous media; Axial dispersion


COMSOL; PDMS; SCMOS; s; ms; μm; mm; kg/m3; v/v; ml; Re; Re+; Pe; μm/s; m2/s


Observations on the paths travelled by nano particles in dilute suspensions, as they are conveyed through models of porous media, are reported in this study. Suspensions of solid particles and liquid droplets in fluids are ubiquitous. As an example, the air we breathe supports numerous microorganisms, viruses, silica, smoke and salt particles, and mist and fog in concentrations that vary with location, time and prevailing conditions such as wind, temperature, humidity and sources of release or production. Concentrations of these particles in air are typically low on a volume fraction basis. On inhalation, such suspended particles pass through and may be captured within the bifurcating generations of narrowing tubes constituting the airways of the lung, a structured porous medium [1].

Physical analogs of porous media with migratory particles abound. Catalytic reactors, hydrocarbon reservoirs and aquifers are systems through the pores of which fine particles are normally displaced and produced with the flowing fluid. As a corollary, tiny tracer particles are injected into porous media to identify potential migration paths of pollutants and contaminants from industrial surface operations or piles of wastes [2]. In the petroleum industry, potential exists to degrade heavy oils and bitumen in-situ, under reservoir conditions, if particles of appropriate catalysts can be transported into partially saturated formations. Commercial and domestic systems and processes involving particles carried in streams through porous bodies include deep-bed filtration in granular or fibrous beds such as in household furnace filters or packed beds for removing white blood cells from platelet-rich-plasma prior to transfusion [3]. It is of interest to identify factors that regulate the rates of particle movements in the conveying fluid, the paths and distances the particles travel, and axial dispersion of the particles as might be correlated to the structures of the porous media to provide the technical foundation for the design of processes or an understanding of the performance characteristics of systems such as described in the foregoing.

The typical porous medium is a complex structure of pores and cavities within arrangements of irregular and opaque particles or fiber strands. Tracking submicron and colloidal particles through such geometry is not often feasible and investigators have attempted to understand the dynamics by resorting to computer simulations [4] and experimental micro models [5]. Geometrically, micro models are two-dimensional representations or thin sections of a porous medium mounted within parallel optically-clear plates. Such an arrangement allows visual observation of fluid flow and particle movement in the interstices between solid elements, and the displacement of one fluid by another. In situations when interfaces are present between immiscible fluids in the medium, the fluid moves in response to local capillary forces and applied pressure. At the macro-scale level, for immiscible displacement, flow through the domain is spatially integrated at the front, even if unstable, and the displacement qualifies as 2-dimensional flow [6]. When particles smaller than the pores are in suspension and their movements are to be tracked, as is of current interest, the flow of the suspending fluid (assumed not a low pressure gas) is described at the microscopic level and the Navier-Stokes equations are applicable. At the pore-level scale, the flow is 3-dimensional for single fluids and miscible displacement, but the irregular and uneven internal boundaries for the flow are difficult to characterize. Generalizations about the local flow dynamics, and influences on particles in motion, are not easy to arrive at unless the structures within the media are regular. It is for these reasons that micro models are often designed as organized arrays of short cylindrical post, typically in triangular or rectangular layout, between two parallel transparent plates.

Highly porous filters are modeled as an ensemble of cylinders acting as collectors [7] or as arrays of infinitely long cylinders oriented parallel or normal to the direction of fluid flow [8,9]. In micro fluidic devices, flow through regular arrays or around cylinders has found application for the separation of particles by size [10]. Simulations have been carried out to predict trajectories for suspended particles but there have been few experimental results to provide validation. A micro-model is valuable for observing and tracking particles [11] but sufficient attention has not been paid to the influence of the flow patterns in the conveying fluid. Di Carlo et al. [10] have reported that fluid and particles interact to cause particles to follow paths unanticipated in micro channels.

Materials and Methods

The apparatus

A schematic of the experimental set up is shown in (Figure 1a). It consists of four components; the test cell, an inverted microscope, image recording and storage devices and an actuating pump. The test cell, viewed from above, is shown in (Figure 1b). It comprises a rectangular channel between two parallel plates that are 28 mm long and 1 mm wide. The spacing between the plates is 20 μm. The bottom plate with sides is fabricated from poly-di-methyl siloxane (PDMS) from silicon wafer molds that have cylindrical holes etched in a region in prescribed patterns by soft lithographic methods [12]. When cured and stripped from the mold, the clear polymer becomes the bottom plate and side walls. A section of the plate has cylindrical posts that are 5 or 10 μm in diameter and uniformly20 μm long projecting out. The posts are in triangular arrays in this study and are within a 1 x 1 mm square area, as shown in (Figure 1b). The top plate of the test cell is a glass cover slip, with injection and withdrawal ports pre-drilled through. The micro model of a porous medium is where the posts are located; and the 10 and 17 mm long sections on either side of this area constitute Hele-Shaw cells through which flows stabilize before entering and after leaving the micro model. Sketches of the arrays of the posts are in (Figure 2a-c). The arrays are identified as configurations I, II and III in subsequent references. It is important to note that the axes of the posts do not form vertices of equilateral triangles. Test cell parameters are listed in Table 1. The solid density is the fraction of the test section occupied by the posts, or 1 minus the porosity. The values were between 0.13 and 0.35 for the configurations. For a typical porous medium, areas ascribed to the bounding walls (if present) are negligible fractions of the total surface area exposed to contacting fluids. This is not the case for the micro models in Table1. Contributions of the bounding surfaces to the observed velocity field that developed and to the viscous resistance to flow in the system cannot be dismissed.