Description
The separation of colloidal particles is of great importance in many fields, such as purification, sensing, and bioanalysis. However, separating particles based on their surface physico-chemical properties remains challenging. Here, we present an innovative microfluidic strategy enabling the continuous separation of colloidal particles based on their surface chemistry. Through experimental and theoretical analyses, we demonstrate that diffusiophoresis and diffusioosmosis enable the separation of carboxylate polystyrene particles with similar sizes and apparent zeta potentials but distinct surface concentrations of carboxyl groups, as well as the separation of protein-coated from bare polystyrene particles.
In the proposed approach, the particles are exposed to salt concentration gradients generated in a double-junction microfluidic device, fed with low and high electrolyte concentration streams [1,2]. A steady-state salt concentration gradient is generated in the transverse (width-wise) direction, perpendicular to the flow. This leads to the accumulation of particles by diffusiophoresis and their migration either outward or inward due to a competition between particle diffusiophoresis and diffusioosmotic fluid flow at the channel walls. As the particles move across environments with varying salinity levels, their dynamics are affected by the sensitivity of their electrophoretic mobility – and consequently, their apparent zeta potential, which is proportional to it – to the local salt concentration.
For low and moderately charged particles, the magnitude of the apparent zeta potential, measured via electrophoretic light scattering, decreases monotonically with the salt concentration, in agreement with Gouy-Chapman theory. For highly charged particles, the dependence of apparent zeta potential on the salt concentration has a non-monotonic trend, due to the electric double layer polarisation and surface conductance. As a result, particles with the same surface charge sign but different charge magnitudes can exhibit opposite sensitivities of apparent zeta potential (and thus electrophoretic and diffusiophoretic mobility) to salt. By harnessing these effects, carboxyl-modified polystyrene particles with low carboxyl group concentrations can be separated with high efficiency from those with high carboxyl group concentrations [3]. Similarly, carboxyl-modified polystyrene particles coated with bovine serum albumin can be fully separated from bare particles [4].
This simple microfluidic approach, which relies on an easy to-operate device with no external energy source, has discipline-spanning potential for the continuous separation of colloids distinguished solely by surface properties such as chemical composition, roughness, permeability, and heterogeneity, that influence the onset of surface conductance within the electric double layer and thus the sensitivities of their apparent zeta potential and diffusiophoresis mobility to the salt concentration. Our microfluidic strategy is particularly promising for point-of-care diagnostics, including bioparticle sensing, sorting, preconcentration, and analysis.
References
[1] N. Singh et al., Physical Review Letters, 125, 248002 (2020).
[2] A. Chakra et al., ACS Nano, 17, 14644-14657 (2023).
[3] A. Chakra et al., Journal of Colloid and Interface Science, 693, 137577 (2025).
[4] C. Puijk et al., Under review (2026)