Ritu R.
Raj
^{a},
C. Wyatt
Shields
IV
^{ab} and
Ankur
Gupta
*^{a}
^{a}Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80303, USA. E-mail: ankur.gupta@colorado.edu
^{b}Biomedical Engineering Program, University of Colorado Boulder, Boulder, CO 80303, USA

Received
26th November 2022
, Accepted 3rd January 2023

First published on 4th January 2023

Diffusiophoresis refers to the phenomenon where colloidal particles move in response to solute concentration gradients. Existing studies on diffusiophoresis, both experimental and theoretical, primarily focus on the movement of colloidal particles in response to one-dimensional solute gradients. In this work, we numerically investigate the impact of two-dimensional solute gradients on the distribution of colloidal particles, i.e., colloidal banding, induced via diffusiophoresis. The solute gradients are generated by spatially arranged sources and sinks that emit/absorb a time-dependent solute molar rate. First we study a dipole system, i.e., one source and one sink, and discover that interdipole diffusion and molar rate decay timescales dictate colloidal banding. At timescales shorter than the interdipole diffusion timescale, we observe a rapid enhancement in particle enrichment around the source due to repulsion from the sink. However, at timescales longer than the interdipole diffusion timescale, the source and sink screen each other, leading to a slower enhancement. If the solute molar rate decays at the timescale of interdipole diffusion, an optimal separation distance is obtained such that particle enrichment is maximized. We find that the partition coefficient of solute at the interface between the source and bulk strongly impacts the optimal separation distance. Surprisingly, the diffusivity ratio of solute in the source and bulk has a much weaker impact on the optimal dipole separation distance. We also examine an octupole configuration, i.e., four sinks and four sources, arranged in a circle, and demonstrate that the geometric arrangement that maximizes enrichment depends on the radius of the circle. If the radius of the circle is small, it is preferred to have sources and sinks arranged in an alternating fashion. However, if the radius of the circle is large, a consecutive arrangement of sources and sinks is optimal. Our numerical framework introduces a novel method for spatially and temporally designing the banded structure of colloidal particles in two dimensions using diffusiophoresis and opens up new avenues in a field that has primarily focused on one-dimensional solute gradients.

Recently, there have been numerous experimental and theoretical reports exploring the motion of active diffusiophoretic particles. These include the effects of finite Peclet numbers,^{19,20} asymmetry in the form of Janus particles and bent rods,^{21–23} changes in the local fluid environment,^{10,13,24,25} and the use of active droplets instead of particles.^{26–28} Such systems have been proposed for uses in applications^{29} such as environmental remediation,^{30} drug delivery,^{31} and cellular transport.^{32}

In contrast to active diffusiophoresis, there are several decades of literature on passive diffusiophoresis. One of the first series of studies to quantify the distribution of colloidal particles under diffusiophoresis was conducted by Staffeld et al.^{33,34} They showed, in electrolytic and non-electrolytic solutes, that the particle distribution exhibits a local maximum, resembling a band that moves with the diffusing solute front.^{33,34} This laid the groundwork for studies of diffusiophoretic banding in other systems, including the well-studied dead-end pore geometry.^{35–38} Experimental studies have been conducted on these dead-end pore systems to optimize nanoparticle transport in collagen hydrogels,^{39} show the size dependence of particle transport into pores,^{40} determine design criteria for particle capture by a pore,^{41} and develop a low cost zeta-potentiometer.^{42} In addition to dead-end pore geometries, similar studies have been conducted in other microfluidic systems. Cross-channel pores have been used to study surface-solute interactions^{17} and the aggregration of colloidal particles near flow junctions.^{43} CO_{2}-induced concentration gradients across microfluidic channels have been used to predict exclusion zone formation in channel flows,^{44} remove bacteria from surfaces,^{45} provide crossflow migration of colloids,^{46} and enable membraneless water filtration.^{47} In a similar way, salt gradients have been used to induce colloidal banding in microfluidic channels.^{16,48}

In addition to the breadth of experimental studies, analytical and numerical techniques have been used to study the phenomena observed in the aforementioned experimental systems. Anderson et al. showed that the diffusiophoretic velocity of a particle is dictated by surface interactions between the solute and particle.^{49–51} For ionic solutes, the diffusiophoretic velocity is given as u_{DP} = M_{e}∇lnc, where M_{e} is the mobility of the particle and c is the electrolyte concentration.^{50} For a particle moving in non-ionic solutes, the diffusiophoretic velocity is given as u_{DP} = M∇c, where M is also a mobility parameter and c is the solute concentration.^{51} These mobility relationships can also be extended to include the effect of multiple ionic species,^{52–54} arbitrary double layer thicknesses,^{55} and ion sizes,^{56,57} amongst others. Numerical studies have been conducted on the spreading of diffusiophoretic particles in response to applied solute gradients with hydrodynamic background flows,^{58} in one-dimensional transient gradients,^{59,60} in concentrated electrolyte solutions,^{61} in solutes that exhibit Taylor dispersion due to a background/diffusioosmotic flow,^{14,62} and in the presence of multiple electrolytes.^{52}

Despite the expansive literature on passive diffusiophoresis, most studies focus on the effects of one-dimensional transient or steady solute concentration profiles on particle motion. The number of studies that expand particle motion to two or three dimensions are limited,^{14,17,41,44,52,62–64} with most focusing on diffusiophoretic motion in two- and three-dimensional channel flows with one-dimensional driving solute gradients.

Recently, Bannerjee et al.^{65} developed “soluto-inertial” beacons that enable them to enact spatio-temporal control over solute gradients surrounding their beacons. This allows them to control and study diffusiophoretic particles moving in response to two- and three-dimensional gradients. They initially designed cylindrical hydrogel posts loaded with sodium dodecyl sulfate that attracted decane droplets and repelled polystyrene particles by releasing solute over a timescale of tens of minutes.^{65} By determining the appropriate diffusiophoretic velocity scale analytically in 3D and numerically in 2D, they were able to collapse the radial dependence of particle velocity.^{65} This proof-of-concept study showed that diffusiophoresis can be used as a mechanism to move colloidal particles deterministically over a length scale of hundreds of microns.^{65} The authors expanded this study to design temperature-triggered beacons, source and sink dipoles, dipoles with distinct solutes, and dipoles with reacting solutes.^{66} In follow-up studies, they developed design principles,^{67} which enabled them to manipulate colloidal distributions in suspension by a sedimenting beacon^{68} and deliver particles to hidden targets.^{5}

Inspired by the work from Banerjee et al.^{66} on source and sink dipoles, we envisioned that multiple solute sources and sinks can be spatially and temporally designed to optimize diffusiophoretic banding in two dimensions. To this end, we outline a numerical procedure for simulating diffusiophoretic colloidal transport in response to a non-electrolytic solute gradient generated by an arbitrary number of point sources and sinks. We determine an appropriate time-dependent molar rate by semi-analytically solving for the flux from a finite-sized solute source. Using our numerical scheme, we determine the timescales governing particle separation in a dipole and octupole source/sink system. For the dipole system, we show that there exists an optimum separation distance between the source and sink that maximizes particle enrichment in a specific region. This optimal distance is set by a balance between interdipole diffusion and molar rate decay timescales. We find that the optimal separation distance depends primarily on the partition coefficient, K, of the source/sink and is weakly dependent on the diffusivity ratio, . Lastly, we show how these principles change the optimal geometric arrangement of sources and sinks in an octupole configuration. Interestingly, we find that the optimal design of an octupole configuration depends on both the spatial arrangement of sources and sinks and the temporal decay of the solute molar rate. These results underscore the rich dynamics observed by expanding diffusiophoretic driving forces to two dimensions. Our results also broaden the potential design space of colloidal banding using diffusiophoresis and provide a numerical framework to study the banding of diffusiophoretic particles in response to an arbitrary arrangement of solute sources and sinks.

We acknowledge that in practical experimental setups, the emission and absorption rates are unlikely to be equal and opposite over time. However, while our numerical framework can handle arbitrary molar rates, we make this assumption to reduce the number of parameters in our system. In addition, we note that u_{DP} as described above uses the non-electrolyte mobility relationship. The rationale to use this relationship is two-fold. First, the non-electrolytic mobility expression does not possess the singularity found in the electrolytic mobility expression. We acknowledge that the singularity can be addressed by considering a concentration dependent electrolytic mobility.^{61,69} For computational convenience, we refrain from incorporating a concentration dependent mobility relation. Second, if the concentration difference is relatively small, the two mobility relationships are equivalent; see Appendix A. Therefore, we choose the non-electrolytic mobility relationship. We acknowledge that there might be quantitative differences if a different mobility relationship is employed, and comment on this difference in Appendix A. Additionally, we acknowledge the limitation in using point sources and sinks, as spatial effects due to the presence of a finite-sized source/sink will yield differences. However, we observe that the qualitative features remain the same as reported in prior experiments;^{66} see Appendix B.

(1) |

We calculate particle motion using two different approaches. First, we use Lagrangian particle tracking to determine the position of particles in time. The center of mass of the i^{th} particle, x_{i}, can be determined by solving the following differential equation

(2) |

Second, we calculate the concentration of colloidal particles, n(r,t). The conservation equation for particle concentration is

(3) |

Before numerically solving, we non-dimensionalize eqn (1)–(3) as

(4) |

(5) |

(6) |

To elucidate the effects of molar rate decay, we use three different scenarios for . First, constant molar rates, where is the strength of the step molar rate and is the heaviside function. In this scenario, there is no timescale associated with molar rate decay and the timescale for colloidal banding is dictated by the interaction between sources and sinks. The second choice of is a boxcar function profile given by where τ_{0} introduces an additional timescale.

Lastly, we derive by calculating the flux emitting from an isolated, finite-sized source of radius a. This allows us to incorporate experimentally relevant parameters, i.e., the partition coefficient of the solute into the source K, and the diffusivity ratio of solute between the source and the bulk . To evaluate we briefly restore dimensions. We assume the origin to be the center of the source. The inner region refers to the concentration field inside of the source, i.e., r ≤ a and the outer region corresponds to the concentration field outside of the source, i.e., r > a. We assume that the concentration in the outer region is initially uniform such that c_{out} = c_{ref}, and the source is saturated with solute such that the concentration in the inner region is c_{in} = Kc_{ref}. At t = 0^{+}, the concentration outside is switched to c_{out} = 0, which leads the source to start emitting solute. The conservation equations for solute inside and outside the source are

(7) |

(8) |

(9) |

(10) |

(11) |

(12) |

is dependent on the partition coefficient K and diffusivity ratio , which we discuss later.

Finite-volume method.
To solve the coupled partial differential eqn (4) and (6), we discretize both equations in space onto a square Cartesian grid with a grid size of 0.05 and write the resulting equations as coupled ordinary differential equations in time. We use a first-order upwinding scheme to resolve the convective term. We implement the point source/sink as a source term in the finite-volume cell, which contains the coordinates for the source/sink. For eqn (4) and (5), we discretize eqn (4) in space and solve the resulting equations with eqn (5) as coupled ordinary differential equations in time. We interpolate the solute gradient at the position of the i^{th} particle during each time step in order to determine the particle velocity. The coupled differential equations are then integrated using an eighth-order Runge–Kutta integration scheme (DOP853) as implemented in Scipy. To gain confidence in our simulations, we compare our results qualitatively to the experimental results of Banerjee et al.^{66} and obtain a good agreement; see Appendix B.

Optimization.
We define an objective function, which inputs the locations of sources and sinks for a given arrangement, solves eqn (4) and (6) with a grid size of 0.1 and outputs a calculated fraction Φ(τ). The fraction is defined as

Φ(τ) represents the fractions of particles within a sub-region Ω_{1} of our domain Ω. We employ the objective function into an optimization scheme to determine a source/sink arrangement that maximizes Φ(τ). The optimization scheme uses a Nelder–Mead simplex algorithm implemented through the Scipy Optimization package.

(13) |

Fig. 2
Dipole simulations for a constant molar rate. (a) Schematic illustration of dipole setup where a source and a sink are separated by a distance d. The shaded region shows the Ω_{1} used in calculating Φ(τ) viaeqn (13) (b–d) _{i}(τ = 0, 50, 100) for 3000 particles as calculated by solving eqn (4) and (5) for d = 3 and = 0.5. (e–g) ñ(,τ = 0, 50, 100), as determined by solving eqn (4) and (6) for d = 3 and = 0.5. The color bar ranges from 0 to 1. All concentration values larger than 1 are truncated to 1. (h) Φ(τ) for a monopole and dipoles with d = 1–6. Continuum results are represented with a solid line while particle tracking results are shown by open circles. Results for a source monopole are plotted in black. (h inset) Φ(τ = 1) for a monopole and dipoles with d = 1–6 in the form of a bar chart. (i) τ_{c}, i.e., the crossover time at which Φ(τ) for the monopole overtakes a dipole with separation distance d, plotted versus d^{2}. The dotted line represents the line of best fit with zero intercept. for all panels. |

Fig. 2(h) (inset) reveals that smaller d values possess a higher Φ(τ) for early times. In contrast, larger d values display a higher Φ(τ) at later times. We also compare these values with the enrichment from a single source, referred here as a monopole. At early times, the monopole provides the least enrichment, Fig. 2(h) (inset). However, at long times, the monopole enrichment surpasses all dipoles. The time at which Φ(τ) of the monopole overtakes Φ(τ) of the dipoles is denoted as the crossover time, τ_{c}. Fig. 2(i) shows a linear trend between d^{2} and τ_{c}. To explain the trends outlined above, we examine eqn (6) more carefully. First, we ignore diffusion as = 10^{−4}. Next, we integrate eqn (6) over Ω_{1} (defined by the shaded region shown in Fig. 2a), and write

(14) |

(15) |

At early times, dipoles have not had sufficient time to interact with each other. Therefore, we argue that to a first approximation, are similar for both a monopole and the source in dipoles. If so, to explain the trend in Fig. 2(h) (inset), eqn (14) implies that at early times, ñ is higher for smaller d values. This appears surprising at first since the from sources and sinks do not interact at this timescale. However, the depletion of particles around the sink increases the concentration of particles at S_{1}, which consequently increases (see Appendix D), leading to a larger Φ.

We argue that dipoles start to interact with each other at τ ∼ d^{2}, or the interdipole diffusion time. For τ ≳ d^{2}, the dipoles screen each other, causing a rapid decline in . After the interdipole diffusion time, becomes localized between the source and sink and diminishes elsewhere. This results in a smaller ; see eqn (15). Since screening occurs later for larger d, the decay in starts later and Φ(τ) is higher; see Appendix D. Finally, for the monopole, screening never occurs, and concentration gradients do not diminish due to interactions with a sink. This is why the monopole overtakes dipoles around the interdipole diffusion time, which results in τ_{c} ∼ d^{2}; see Fig. 2(i).

The aforementioned discussion highlights the time-dependent nature of enrichment. Therefore, we seek to study the effects of a time-dependent molar rate. To this end, we employ a molar rate profile given by where is the Heaviside function; see Fig. 3(a). This molar rate provides us with two parameters: the strength of the molar rate and the time for the molar rate to decay to zero τ_{0}. Fig. 3(b) shows Φ(τ) for τ_{0} = 18.2 and d = 1–6. The choice for τ_{0} corresponds to the crossover time observed in Fig. 2 for d = 3. For τ > τ_{0} (represented by the dashed line in Fig. 3(b)), Φ(τ) increases slightly before leveling. At τ = τ_{0}, we also observe that Φ(τ) increases with separation distance until d = 3 and then slightly decreases. Thus, there is an optimal separation distance. Using the described optimization scheme, we determined the optimal separation distance, d_{opt} as a function of τ_{0} and . In Fig. 3(c), we observe that a plot of d^{2}_{opt}versus τ_{0} results in a linear trend. Additionally, from Fig. 3(d), we see that d_{opt} is weakly dependent on .

The d_{opt} is set by a balance between the interdipole diffusion and molar rate decay timescales. This is seen by the linear trend between d^{2}_{opt} and τ_{0} observed in Fig. 3(c). When the source and sink screen each other before the molar rate is turned off, leading to small Φ(τ). When the enrichment around the source is boosted due to depletion around the sink, however, the source and sink do not screen each other as the molar rate vanishes at the interdipole diffusion time. Finally, when the enrichment around the source is less impacted by the depletion around the sink. In effect, becomes the optimal distance. In summary, the timescale of molar rate decay can be used as a parameter to optimize particle enrichment.

and are not easy to realize experimentally. Instead, as shown by Banerjee et al.,^{65–67} solute fluxes arise due to solute partitioning between source and the bulk, described by a partition coefficient, denoted here as K. We also define the diffusivity ratio, , as the ratio of solute diffusivity in the source and in the bulk. As such, we incorporate the effects of these parameters by determining using eqn (12). Fig. 4(a) shows for different values of K and . As expected, the molar rate has a higher strength for a larger K value, and the decay is slower for a smaller value of .

Fig. 4
Optimal separation distance for experimentally realizable
. (a) as calculated by inverting eqn (12), for a finite-sized source of radius . K = 10, 1000 and = 10^{−1}, 10^{−3}. (b) Φ(τ) for K = 100 and = 10^{−2}, d = 1–8. (c) d_{opt}vs. for K = 500. (d) d_{opt}vs. K for = 10^{−2}. |

We conduct dipole simulations by solving eqn (6) with determined by inverting eqn (12). We evaluate Φ(τ) for different values for K and . Fig. 4(b) shows Φ(τ) with and for different d values. Much like Fig. 3, we observe an optimal separation distance, d_{opt} ≈ 5. This demonstrates that d_{opt} is a generic feature of a time-dependent molar rate. We investigate the dependence of d_{opt} on K and using the optimization scheme described earlier. Fig. 4(c) shows the variation of d_{opt} with for K = 500, where we observe that d_{opt} is weakly dependent on . However, Fig. 4(d) shows that d_{opt} is strongly dependent on K.

The result of d_{opt} showing a weak dependence on is surprising, as one would expect to impact the timescale of solute molar rate decay, which would ultimately impact the optimal separation distance. Therefore, we investigate this effect further. We note that there are two timescales for a short timescale, during which solute transport occurs over a small boundary layer within the source, and a longer timescale where concentration gradients inside of the source are fully developed. An expansion of eqn (12) around large s (small τ) shows that

(16) |

(17) |

Given our understanding of timescales and their impact on optimal banding in dipole systems, we seek to expand our work to probe how the geometric arrangement of four sources and four sinks around a circle of radius , termed here as an octupole system, affects banding. Fig. 5(a) shows the four octupole arrangements we study. Case 1 refers to the arrangement where each source is nearest to two sinks and vice versa, i.e., a relatively symmetric arrangement. Case 4 refers to the most asymmetric scenario where four sources are arranged consecutively, followed by four sinks. Case 2 and Case 3 are in between, with Case 2 being more symmetric than Case 3. The shaded areas outlined by dashed lines represent the integration region that Φ(τ) is calculated over. Fig. 5(b and c) show simulation snapshots at τ = 100 for with (panel b) and (panel c). τ_{0} = 18.2 is used for all simulations.

Fig. 5
Geometric and spatial effects on banding for an octupole. (a) Four arrangements studied in an octupole system with the shaded regions showing the Ω_{1} used in calculating Φ(τ) viaeqn (13). The sources and sinks are placed around a circle of radius . (b and c) Simulation snapshots at τ = 100 with for and . (d) for Cases 1–4 with varying from 1–5. |

We quantify i.e., the relative increase in Φ. Fig. 5(d) shows η for all four octupole arrangements, with varying from 1 to 5. For Case 1 experiences the smallest increase in Φ(τ), while Case 4 experiences the largest increase. As increases from 1 to 5, this trend reverses and Case 1 experiences the largest increase in Φ(τ) while Case 4 experiences the smallest increase. To understand this trend, we invoke our understanding from the dipole arrangement. The octupole has multiple interpole diffusion timescales. The smallest timescale is associated with and the longest timescale is associated with . When the maximum . Therefore, all sources and sinks interact before the molar rate decays. In this scenario, the arrangement with the most geometric asymmetry, i.e., Case 4, has the largest η. Intuitively, in this case the source/sink screening is minimized, as the sources and sinks are collectively the furthest apart. When the smallest implying that none of the sources and sinks interact. Case 1 performs best in this regime, as sources are able to benefit from a local increase in ñ(,τ) due to depletion from multiple nearby sinks. This effect is similar to the increase in performance for dipoles compared to a monopole observed earlier, see Fig. 2(h). Lastly, we note that η, for all four cases, increases with because d_{ij} also increases with . As increases, the sinks and sources enrich particles for longer before interacting. We underscore that such complex banding patterns are unlikely to occur in one-dimensional diffusiophoretic systems as the motion of colloidal particles is restricted to one direction.

Looking forward, our results provide design principles for engineering microfluidic devices^{5,65–67} that utilize diffusiophoresis to move colloidal particles and create banded patterns. By utilizing partition coefficients and spatial arrangement, one can impart temporal and spatial control over the banded structure of colloidal particles. From a fundamental perspective, our results can also be expanded to include flow effects such as dispersion due to diffusiophoresis or diffusioosmosis.^{14,37,52,58,62,71,72} Additionally, there is the potential to use such a system for applications that require precise control over colloid localization, such as biosensing,^{73} colloids separation,^{74} and two-dimensional micropatterning.^{75} Dipole and octupole systems, as envisioned, could be created using lithography similar to ref. 66. Our work also invites future studies that move away from point sinks and sources, include higher-order effects and investigate asymmetric fluxes between sources and sinks. The results, as outlined in this article, motivate future experimental and theoretical studies to investigate two- and three-dimensional diffusiophoretic banding.

(18) |

(19) |

If the concentration difference is significant compared to the background concentration, the electrolytic and non-electrolytic expressions will yield a different response. Specifically, for an electrolytic mobility expression, the additional dependence will yield a higher u_{DP} around the sink. In contrast, u_{DP} will decrease around a source. We anticipate the qualitative features will remain the same. We invite interested readers to explore this effect quantiatively in future studies.

Fig. 6
Comparison with experimental work by Banerjee
et al.
^{66} (a) Example of particles moving in response to gradients generated from a source and sink, reproduced and adapted from ref. 66 with permission under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). (b) Particle streaklines showing time-coded trajectories for particles with = −0.5. d = 3 and . Simulation results are for ,ỹ ∈ [−10,10], but are zoomed in to ,ỹ ∈ [−3,3]. |

(20) |

(21) |

We drop the tildes for convenience. We now have a set of two ordinary differential equations. We substitute in eqn (20) and obtain

(22) |

(23) |

(24) |

(25) |

(26) |

A(s) will be determined when applying the partition and flux-matching boundary conditions. Returning to the outer problem, we write eqn (21) in terms of a modified Bessel's equation

(27) |

(28) |

(29) |

(30) |

(31) |

(32) |

(33) |

(34) |

We write our expression for ĉ_{in} and ĉ_{out} as

(35) |

(36) |

Lastly, we find an analytical expression for the flux at the interface between the inner and outer region as

(37) |

Fig. 7
for
= 0.5, . for a monopole (black line) and dipoles with d = 1 − 6 for a constant molar rate . |

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