(732e) Particle Deposition On Patchy and Janus Collectors: Design of Artificial Filter Systems with Improved Life Cycle

Chemically patchy surfaces are ubiquitous in nature and encompass a wide gamut of applications

from biomedial research to nanotechnology. State-of-the-art fabrication technology has enabled

facile production of particles with chemical anisotropy in bulk. Janus particles have been used as

bimetallic spherical motor particles to mimic commonly occurring nanoscale biomotors present

in biological systems. Other biological applications include adsorption of molecules and proteins

on self assembled heterogeneous arrays. Interestingly such chemically heterogeneous surfaces

are widely encountered in porous media flows as well. In light of all these applications, it is

important to develop a theoretical model to analyze the deposition behavior of colloidal particles

in a porous medium with such patchy collectors. In this present study, we describe methods how

such patchy and Janus collectors could be used to design fluidized field collimated variable

porosity filter with improved operational life cycle.

An Eulerian model (convection-diffusion-migration equation) is presented to study the

deposition behavior on Janus and Patchy spherical collectors using Happel Cell geometry. The

model aims to capture the effect of surface charge heterogeneity on the particle deposition rate

on these spherical collectors. Two separate cases of surface charge distribution is presented. In

the first case, the surface heterogeneity is modeled as half the collector favoring deposition and

the other half hindering it (Janus collectors). For the second case, the surface heterogeneity is

modeled as alternate stripes of attractive and repulsive regions on the collector (Patchy

collectors). The model also considers fluid flow approaching the collector at different angles in

addition to the standard gravity assisted and gravity hindered flow conditions to analyze the

effect of collector orientation on the deposition.

In most naturally occurring substrates, the surface charge is randomly distributed and difficult to

characterize. In order to render systematic analysis tractable, the charge heterogeneity on the

collector surface is modeled as alternate stripes of positive and negative bands. Thus, there is

alternate attractive and repulsive interaction due to the DLVO interactions between the particle

and the collector. The particle concentration around the collector is studied and particle

deposition behavior is then related to fraction of the surface area favorable for deposition.

The particle concentration distribution and the corresponding overall deposition rates are

presented for two types of surface patterning for charge heterogeneity as mentioned earlier. The

cases discussed are for a Janus collector and a patchy collector with micropatterned surface

charge distribution. In both cases the results indicate particle accumulation at the leading edge of

the favorable stripe due to tangential transport of particles over the unfavorable section of the

collector. The results also indicate the presence of a small inaccessible part on the favorable

stripe at distances further away from the forward stagnation line. For a sphere-in-cell geometry,

the model shows the relative insignificance of lower half of the collector playing a dominant role

in the deposition process. For Janus and patchy collectors, the effect of different flow

orientations was also analyzed. The results indicate the possibility of designing filter beds using

these heterogeneous collectors which can have longer operational life compared to homogeneous

collectors. Such collectors can also be used for protein adsorption in biological systems. For a

micropattened charge surface, the comparison of the overall deposition rates with patchwise

heterogeneity model shows significant deviation. The assumption that deposition rates on the

favorable patches of the collector are unaffected by the charge heterogeneity deems the

patchwise heterogeneity model inaccurate for micropatterned surfaces. The model also shows the

dependence of the overall deposition rates on particle size and fluid flow velocity.

Designing a filter bed with patchy collectors can have added benefits compared to that of a

homogeneous collector. It has already been shown that a patchy collector can have deposition

characteristics almost identical to that of homogeneous collectors under given conditions.

However, using external fields the collector can be flipped just as discussed for Janus collectors.

For a system containing only one type of charged particles, this would be useful as the cleaner

lower half can now be employed to capture the particles. A filter bed with such collectors will

have longer operational life compared to a homogeneous collector.