Modulation of Spatiotemporal Particle Patterning in Evaporating

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Modulation of Spatio-temporal Particle Patterning in Evaporating Droplets: Applications to Diagnostics and Materials Science Rajarshi Guha, Farzad Mohajerani, Ahana Mukhopadhyay, Matthew Collins, Ayusman Sen, and Darrell Velegol ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13675 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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ACS Applied Materials & Interfaces

Modulation of Spatio-temporal Particle Patterning in Evaporating Droplets: Applications to Diagnostics and Materials Science Authors Rajarshi Guha,a Farzad Mohajerani,a Ahana Mukhopadhyay,a Matthew D. Collins,b Ayusman Sen,b,* Darrell Velegola,*

Affiliations a

Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, U.S.A.

b

Department of Chemistry, Pennsylvania State University, University Park, PA 16802, U.S.A.

Email: [email protected], [email protected]

Abstract

Spatio-temporal particle patterning in evaporating droplets lacks a common design framework. Here, we demonstrate autonomous control of particle distribution in evaporating droplets through the imposition of a saltinduced self-generated electric field as a generalized patterning strategy. Through modeling, a new dimensionless number, termed “capillary-phoresis number” (CP), arises which determines the relative contributions of electrokinetic and convective transport to pattern formation, enabling one to accurately predict the mode of particle assembly by controlling the spontaneous electric field and surface potentials. Modulation of CP allows the particles to be focused in a specific region in space or distributed evenly. Moreover, starting with a mixture of two different particle types, their relative placement in the ensuing pattern can be controlled, allowing co-assemblies of multiple, distinct particle populations. By this approach, hypermethylated DNA, prevalent in cancerous cells, can be qualitatively distinguished from normal DNA of comparable molecular weights. In other examples, we show uniform dispersion of several particle types (polymeric colloids, multiwalled carbon nanotubes and molecular dyes) on different substrates (metallic Cu, metal oxide and flexible

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polymer) as dictated by CP. Depending on the particle, the highly uniform distribution leads to surfaces with lower sheet resistance, as well as superior dye-printed displays.

Keywords: particle distribution; capillary-phoresis number; convection; electrokinetics; separation; methylated DNA.

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1.


Introduction The spatio-temporal control of particle dispersion is an important issue in diagnostics and materials

science. In applications like spray painting, inkjet printing, and thin film deposition,1 a uniform dispersion of particles is critically important. On the other hand, nanostructured self-assembly2 and lithographic patterning3,4 requires the assembly of particles in specific locations in space. Additionally, many diagnostics applications require either particle assembly in specific locations5,6 or uniform dispersion.7,8 Particles in evaporating droplets tend to accumulate at the edges, the so-called “coffee ring” effect. Coffee rings can be broken up by the addition of surfactants, but the distribution of particles remains non-uniform and patchy.9–12 As a special case, coffee ring formation can be suppressed by using high-aspect ratio particles.13 There are, however, no general guiding principles for controlling particle distribution in evaporating droplets. Here, we demonstrate autonomous chemically-driven electrokinetic flows as a general method for controlling particle placement both at outer band (“ring” area) and on the entire wetted area. More fundamentally, particle patterning is guided by a dimensionless “capillary-phoresis number” (CP) which is easily evaluated. Previous reports on sessile droplet evaporation11,13–16 did not study salt generated electric fields and more specifically, diffusiophoretic or diffusio-osmotic fluid flows.17 Additionally, higher salt concentrations (≥10 mM) used in some studies resulted in particles sticking to the substrate surface18,19 without much control on the resulting pattern and the underlying electrokinetic phenomena was not discussed.17 Circular bands of particles (“coffee rings”) form during the evaporation of droplets of colloidal suspension due to evaporation-induced capillary convection.14 We find, that is possible to counteract this process by engineering an opposing electric field through the addition of specific salts at millimolar concentrations which causes diffusiophoretic and diffusio-osmotic flows in the system Faster water evaporation from the thin liquid layer near the contact line causes a rapid increase in ionic concentrations there. The resulting radial salt concentration gradient produces a spontaneous electric field within the droplet that causes electrokinetic transport of the particles and of the fluid near the substrate surface. By choosing salts for which 3



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the diffusivity of either the cation (D+) or the anion (D-) is higher, we can control whether the particles are deposited in an intense circular band, a broken circular band, or dispersed throughout the evaporating droplet. Our experiments are supported by modeling, from which emerges the dimensionless CP that determines the relative contributions of electrokinetic and convective transport to pattern formation. While reports on particle assembly overwhelmingly focus on a single particle type, co-assemblies of multiple, distinct particle populations are critical to the bottom-up design of multifunctional devices.20–22 Starting with a mixture of two different particle types, we can dictate their relative placement in the ensuing pattern based on CP. In a striking example of autonomous chemically-powered separation, we were able to separate positively charged particles at the outer band with >90% enrichment from the corresponding particles of opposite charge but identical size, without the use of an externally applied field. Taking advantage of the difference in correlation/ persistence lengths of normal and hypermethylated DNA of comparable molecular weights,23 we were able to readily distinguish the two at aM concentrations without requiring specialized equipment,24–27 detail sequencing28 or specific antibodies.29 In contrast, to illustrate the general applicability of our technique in obtaining uniform dispersion, we show that different materials (polymeric latex particles and multi-walled carbon nanotubes) can be evenly dispersed on different substrates (metal, metal oxide and plastic), as dictated by CP. As an example, we demonstrate uniform coatings with molecular dyes (e.g. Rhodamine 6G) for printing and display applications, which cannot be obtained using currently practiced procedures involving Marangoni effect.30 2.

Results and Discussion

2.1. Electric field driven diffusiophoresis: Effect of salt β

In a saline droplet resting on a substrate surface, faster evaporation at the edge produces a higher salt concentration near the contact line (see Figure 1A). Constituent ions back-diffuse to the interior of the droplet from the contact line and the resulting radial concentration gradient causes spontaneous chemically-driven diffusiophoretic transport of colloidal particles, with typical speeds of 0.1 to 1 µm/s.31,32 Thus, the chemical 4


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energy derived from concentration gradients is transduced into mechanical motion of suspended colloidal particles. Diffusiophoresis has two components: electrophoresis and chemiphoresis. The electrophoresis part derives from a concentration gradient of cations and anions with different diffusivities (D+≠D-), giving rise to a spontaneous electric field E = β kT ∇ c / Zec , where β = (D + − D − ) / (D + + D − ) is a dimensionless diffusivity parameter for a Z:Z binary symmetric electrolyte, kT is the thermal energy, e is the proton charge, c is the local salt concentration in radial direction, and ∇ is the gradient operator in cylindrical co-ordinates. A gradient of NaCl (β = -0.207, a “β negative” salt) gives a much larger E field than the same gradient of KCl (β = -0.02 since DK+ ≈ DCl-). Depending on whether the salt is β negative (β-) or β positive (β+ e.g. potassium hydrogen phthalate, KHP, β = +0.65, and potassium benzoate, PB, β = +0.47), the electric field points either inward or outward (Figure 1A, drawn for PB/ KHP). Importantly, this E field also acts on the underlying substrate surface and particles, giving rise to diffusioosmotic fluid flow and electrophoretic particle transport. Chemiphoresis, on the other hand, is due to gradient-induced asymmetric pressure in electrical double layer, resulting in fluid flow from the areas of high to low concentrations. Typically, electrophoresis dominates over chemiphoresis in our systems, except when β is small as in the case of KCl. 2.2. Comparison between NaCl and KHP: Particle transport and outer band structures Systematic experiments were done to examine pattern formation in the presence of various salts with sulfated polystyrene latex particles (sPSL, diameter 4.0 µm, ζp = -89.0 mV at 2 mM NaCl) under controlled conditions (25±1˚C, 35% relative humidity). In all experiments, the droplet was pinned at the contact line, but the droplet height and the contact angle decreased with progression of evaporation. The particles move due to (a) the capillary convection within the droplet (Uconv), (b) diffusioosmotic transport of fluid along the underlying substrate surface (vdo), and (c) diffusiophoretic transport of the particles in the salt gradient (Udp). Figures 1B-1C describe the different patterns generated with 2 mM NaCl (thicker and compact ring) and 2 mM KHP (thinner and broken ring). The arrows in the figures point out the same and opposing directions for Uconv and Udp that exist in NaCl and KHP systems, respectively, resulting in such characteristic patterns. Note that 5



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modeling suggests that the magnitude of vdo is significantly smaller than Udp due to lower value for the wall zeta potential (ζw) compared to particle zeta potential (ζp). We then compared experimentally-tracked speeds of tracer particles with the modeled total velocities (Utotal) that include Uconv, Udp and vdo (Figures 1D-1F), with the experimental data supporting the model (see Supporting Information, Supporting Figures S1-S4, and Supporting Videos S1-S3). It is to be noted that Peclet number (Pe) for the ions in the evaporative system was only ~ 0.1 in the interior of the droplet at around half evaporation time and, therefore, we modeled diffusiophoresis and convection components separately. Additionally, all previously mentioned flows in the system are measured and modeled in radial direction (r) from the center of the droplet. 2.3. Electrokinetics in evaporating drops: Definition of capillary-phoresis number (CP) In order to understand our experiments quantitatively, we modeled our experimental systems using capillary convection results from the literature,33 and included electrokinetic contributions (see Supporting Information). The modeling can be summarized by using a dimensionless number, capillary-phoresis number (CP), described below. This number represents electrokinetic to convective velocity ratio with exact “0” value corresponds to DI water. The diffusiophoretic particle speed, along r direction, is given by:

U dp =

 Zeζ p  ∇C(r ) εkT  2kT   ln 1 − tanh 2  βζ p − ηZe  Ze   4kT  C(r )

(1)

Where ε is the dielectric permittivity and η is the viscosity of the medium, Z is the valence of a Z: Z salt, ζp is the particle zeta potential, C is the local solute concentration, and ∇C is its gradient in r direction. The first term in brackets is due to electrophoresis, while the second term is due to chemiphoresis. Approximating that ∇C/C ~ 1/R and Uconv ~R/ te, where R is the droplet radius and te is the total droplet evaporation time (~1000s for 5.0 µL droplet), and assuming that electrophoresis (first term in equation 1) dominates over chemiphoresis, we define the dimensionless number:

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CP =

Uel εkTte = β (ζ p − ζ w ) Uconv ZeηR 2

(2)

where the total electrokinetic velocity in r direction, Uel = Udp + vdo, and ζw denotes wall zeta potential. The time averaged value of ∇C/C inside a droplet is close to the value of 1/R, despite the fact that ∇C/C is a spatiotemporal quantity. The CP accounts for 1) the β value of the salt and, therefore, the direction of the E field, 2) the zeta potential of the particle (ζp), which governs its electrophoretic transport, and 3) the zeta potential of the wall (ζw), which causes electroosmotic fluid movement. CP is essentially independent of droplet size, since te/R2 is roughly constant (see Supporting Information). CP is the combined effect of both β and difference between (ζp-ζw), which renders it non-zero even for salts with low β values (e.g. KCl). CP = 0 signifies Uel = 0 which is the case in DI water as explained by Deegan et al.14 By using substrates of lower magnitude of ζw, such as metallic copper surface or ITO (Indium tin oxide) coated glass surface (Supporting Table S1), diffusioosmotic fluid flows can be minimized resulting in more negative CP and better particle dispersion. In essence, the dimensionless CP determines the relative contributions of electrokinetic and convective transport, and predicts the nature of the pattern formed. From modeling, we estimated that the salt concentration near complete evaporation stays well below 1 M and therefore, viscosity increase can be neglected in our system. Additionally, the contact angle of the droplets did not vary with different salts and all the measured values were comparable to DI water (Supporting Figure S3C). Therefore, we ruled out any significant surface tension effect due to salt addition. Overall, CP is a relative quantity which is suitable for comparing different salts, particle and substrate surface potentials. We observed that the correlation of CP value with outer band thickness does not change significantly within a range of volume fractions (φ) (Supporting Figure S4). However, CP does not capture particle size, settling or electrostatic interaction between oppositely charged particles and surfaces.34 We mainly used higher φ (~ 10-4) in case of larger particles (1- 4 µm) and smaller φ (~10-5) in case of smaller particles ( KIO3 > KH2PO4 > PB > KHP. [Bmim]Cl (1-butyl-3-methylimidazolium chloride) is an ionic liquid. (B) Average breakage coefficients (BC: length of unoccupied regions to the total length in band sections) correlates with the salt and charge of sPSL particles, defined by the CP. The inset figures were taken in fluorescence mode. The error bars represent standard deviation over at least three independent experiments. Scale bars represent 60 µm. (C) Peripheral band formed by 0.1 µm green fluorescent negatively-charged carboxyl modified polystyrene latex particles (cPSL) in 2 mM NaCl as seen under laser scanning confocal microscope was thicker than the band formed in 2 mM KHP (E). (D) Band formed with 0.1 µm red fluorescent positivelycharged amine modified polystyrene latex (aPSL) in 2 mM NaCl was much thinner than in 2 mM KHP (F). (CF) demonstrates that reversing particle charge reverses the peripheral ring thickness in accordance with our model. The particle volume fraction for C-F was 6.3×10-5. The scale bars are 100 µm.

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Figure 3. Separation of differently charged particles of identical sizes by selective banding modulated by CP. (A) 1.0 µm cPSL (purple, negatively charged) particles were separated from a mixture with oppositely charged 1.0 µm aPSL (blue, positively charged) particles (φ = 7.9 × 10 each) at the peripheral band by 29



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evaporating 3 µL droplets with appropriate salts which modulate the CP. Confocal images showing relative distribution of cPSL particles in peripheral band and in the interior with 2 mM NaCl (CP = +0.052) (B-C), 2 mM KHP (CP = -0.161) (D-E), and 5 mM KHP (CP = -0.225) (F-G). As seen, the population of cPSL particles was reduced at the outer ring with decreasing CP, leaving behind only the aPSL particles in the band. The scale bars in B, D and F are 250 µm and the scale bars in corresponding zoomed in images- C, E and G are 50 µm. (H) Measured total intensities of cPSL particles in the band with 2 mM NaCl, 2 mM KHP and 5 mM KHP, respectively (left y-axis). Reduction of ~40 fold in cPSL intensity was obtained by using 5 mM KHP as compared to 2 mM NaCl. This reduction results from the movement of cPSL particles to the interior (Supporting Figure S6) owing to inward diffusiophoretic transport dictated by CP. On the right y-axis, % aPSL in a mixture of aPSL and cPSL particles at the band, increased to 95.0±4.0% with 5 mM KHP compared to 70.0±3.0% with 2 mM NaCl addition. The dotted line is guide to the eye.

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Figure 4. Distinguishable outer band patterns based on DNA correlation lengths () is facilitated by CP. Schematic of tandem interactions consisting of electrokinetic and depletion effects resulting in more aggregated and less aggregated band patterns. Tracer particles were driven by positive CP (+0.1237 for 2 um cPSL particles in 3 mM KHP) to the outer band where they rearrange according to depletion interactions dictated by either nonmethylated or methylated DNA ξ (Supporting Information). DNA deposition and aggregation pattern was demonstrated using composite image of YOYO-1 labeled green fluorescent lambda phage DNA (λ-DNA) and cPSL particles.

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Figure 5. Differentiable outer band patterns form in presence of λ-DNA methylation. (A-B) Intense outer band resulted from evaporation of a mixture of 2 µm cPSL particles (φ = 3.0 × 10 ) and 0.5 nM nonmethylated λ-DNA diluted in 3 mM KHP. The scale bars are 250 µm and 50 µm in B and C, respectively. (C-D) The outer band structure became less aggregated due to the presence of 0.5 nM methylated lambda phage DNA (me-λ-DNA) diluted in 3 mM KHP along with 2 µm cPSL particles (φ = 3.0 × 10 ).

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Figure 6. Nonmethylated DNA gives denser particle arrangement at the outer band while hypermethylated DNA gives dispersed particle arrangement as measured by mean gray value (a.u.). (A) Detection of me-λ-DNA by measuring the lower mean gray value at the outer band at different DNA concentrations (0.5 nM and 1 nM) in 3 mM KHP. In contrast, detection in DI water was not possible due to smaller particle population at the outer band owing to the absence of favorable electrokinetic transport. (B) Detection using universal nonmethylated and hypermethylated human DNA (15 µg/ ml or 7 fM) diluted in 3 mM KHP. Nonmethylated DNA formed significantly more aggregated outer band compared to methylated DNA (me-DNA). (C) The presence of 23.25 aM (50 ng/ ml) hypermethylated DNA in a mixture of hypermethylated (50 ng/ ml) and nonmethylated (50 ng/ ml) DNA can be detected by comparing with 46.5 aM (100 ng/ ml) of nonmethylated DNA. (D) Carcinogenic Jurkat genomic DNA is detected due to less aggregated outer band (altered methylation pattern) as compared to normal genomic DNA from healthy individuals. The error bars are based on standard deviation of n = 5-10 sample size.

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Figure 7. Uniform dispersion of different particles (dielectric: sPSL particles; conductive: MWCNTs) on different substrates (metal: Cu; metal oxide: ITO; plastic: PET) dictated by CP. (A) Particle distribution was non-uniform with thick banding in DI water. 3 µL drops of 4.0 µm sPSL particles with φ = 7.5 × 10 was evaporated on metallic Cu surface. (B) Banding was eliminated and the internal particle distribution was more uniform with 5 mM KHP on Cu surface with CP ~ -0.26. The observed striations are due to inherent roughness of the copper substrate. The scale bars are 100 µm. (C) Strong outer band with sparse deposition in the inner area on conductive ITO surface resulted from evaporation of 100 µL droplet containing 0.01% MWCNT 34


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dispersed in DI water. (D) 0.01% MWCNT was evenly distributed on ITO surface with 5 mM KHP (CP ~ 0.15). Scale bars are 1 mm. Evaporation of 1 ml droplet of 0.01% MWCNT dispersion on thin flexible PET films (100 µm thickness) demonstrate non-uniform distribution with DI water (E) and uniform dispersion with 5 mM KHP driven by CP ~ -0.05 (F). Scale bar, 1 mm. (G) 5 mM KHP dispersed MWCNTs on thin flexible PET films make conductive surfaces (Rs ~ 41.6 kΩ/ square) with more than order of magnitude less sheet resistance than those obtained using DI water dispersed MWCNTs (Rs ~ 19.0 MΩ/ square).

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Figure 8. Uniform dispersion of Rhodamine 6G on flexible polymeric substrate using chemically-driven electrokinetic flows. (A) Drop printing of letter “P” using 1 mM Rhodamine 6G in DI water on flexible polymer sheet resulted in contact line depinning and dye aggregation. (B) Drop printing of letter “P” using 1 mM Rhodamine 6G on flexible polymer sheet with 5 mM KHP added to the ink. (C) Non-uniform and patchy “P” resulted when 5 mM surfactant SDS was added to the ink. (D) Closer look on individual droplet shows nonuniform distribution in DI water and (E) uniform distribution of the dye with electrokinetic effect. The confocal image in the inset clearly shows no banding and improved dye distribution. (F) Strong outer band formation with patchy distribution of the dye due to Marangoni effect as observed in the inset confocal image. Note the circular white patterns on the transparent polymeric sheet were pre-existing. The scale bars are 1 mm.

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ToC Graphic for manuscript

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Modulation of Spatiotemporal Particle Patterning in Evaporating

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