Description
Many medically and environmentally relevant pathogens, including viruses and bacteria, exhibit pronounced shape anisotropy, ranging from spherical to elongated geometries. Their removal in pharmaceutical manufacturing and water treatment commonly relies on membrane-assisted depth filtration. Despite its widespread use, a mechanistic understanding of how membrane surface chemistry and particle geometry govern retention remains limited. Direct in-situ visualization of single particle behavior within real filtration membranes is highly complex. Consequently, mechanistic insights are largely derived from microfluidic membrane-mimicking devices (MMD) using spherical model particles. This raises the fundamental question: to what extent are these results transferable to the transport and capture of non-spherical colloids?
Here, we address this question using a microfluidic depth filtration MMD that enables systematic control over collector surface chemistry and direct visualization of particle dynamics. We investigate (i) the role of surface chemistry in depth filtration and (ii) the extent to which filtration behavior depends on particle shape. First, transport and retention of spherical poly(ethylene glycol) diacrylate (PEGDA) particles are quantified on polydimethylsiloxane (PDMS), oxygen-plasma-treated PDMS, and Poly(diallyldimethylammonium chloride) (PDADMAC)- and Poly(styrenesulfonic acid) (PSS)-modified collector surfaces. Identical filtration conditions are then applied to rod-shaped PEGDA particles with aspect ratios of 1.5 and 3 on PDMS and PDADMAC collectors. Across all particle geometries, the minimum particle diameter is kept within a comparable range.
For spherical particles, we found that filtration performance is strongly governed by surface chemistry. However, hydrophilicity alone is insufficient to predict fouling behavior. Surfaces with similar charge but differing wettability exhibit different flux declines. Instead, surface charge and specific collector–particle interactions emerge as the dominant parameters of retention. Breakthrough analysis reveals particle passage on hydrophilic surfaces, whereas hydrophobic PDMS achieves complete retention, underscoring the nontrivial interplay between wettability and electrostatic interactions in depth filtration.
In contrast, for rod-shaped particles, trends established for spherical colloids are not directly transferable. Breakthrough behavior and flux decline are insensitive to surface chemistry and remain within a comparable range across collectors. Particle geometry instead emerges as the dominant parameter. Surface chemistry primarily influences pore-scale particle orientation, broadening the distribution of orientations relative to the flow direction. Increasing particle aspect ratio leads to reduced flux decline and lower breakthrough, as elongated particles are retained earlier within the porous structure.
Collectively, these results demonstrate that particle shape fundamentally alters filtration mechanisms, limiting the predictive value of spherical model systems for anisotropic pathogens and highlighting the need to explicitly account for particle geometry in the design and interpretation of depth filtration processes.