Letter
www.acsami.org

Hybrid Self-Assembly during Evaporation Enables Drop-on-DemandThin Film Devices
J. William Boley,*,†,§ Seok-Hee Hyun,‡ Edward L. White,† David H. Thompson,‡and Rebecca K. Kramer*,††

School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States‡

S Supporting Information*

ABSTRACT: We propose and demonstrate a hybrid self-assembly
process as the mechanism for producing strikingly uniform deposits
from evaporating drops composed of cosolvents. This assembly
process leverages both particle−ﬂuid interactions to carry the particles
to the drop surface and particle−interface interactions to assemble theparticles into a uniform ﬁlm. We anchor our results in a cosolvent
evaporation model that agrees with our experimental observations. We
further employ the process to produce thin ﬁlm devices such as ﬂexible
broadband neutral density ﬁlters and semitransparent mirrors. Our
observations suggest that this assembly process is free of particle−
substrate interactions, which indicates that the results should betransferable across a multitude of material/substrate systems.
KEYWORDS: liquid metal, gallium−indium, self-assembly, drop evaporation, cosolvent, monolayers, thin ﬁlms, opticsA

formation. Moreover, this technique is used to produce thinﬁlm devices such as ﬂexible broadband neutral density ﬁltersand semitransparent mirrors. This hybrid self-assembly
approach requires no particle−substrate interactions, is scalable,robust, and transferrable. In essence, this work reveals themechanism for co-solvent-based uniform deposits and demonstrates applications for drop-based fabrication of thin ﬁlm
devices.
This paper describes a process that is applicable to any
cosolvent system where the dispersed particles (or nonvolatilematerial, in general) are miscible only in the solvent with ahigher vapor pressure. However, the scope of this study is
focused speciﬁcally on a system composed of an ethanol/water
cosolvent mixture, which has been well-studied,10,13−15 andthiol-capped Ga−In nanoparticles (see Figure 1a), a recently
characterized class of nanoparticles.16,17 The process istransferable to any substrate on which there is contact line
pinning throughout evaporation; however, for this study wefocus our results on polydimethylsiloxane (PDMS) substrates.
We found substantially varying deposit outcomes with othercontrol groups (see Supporting Information Figures S1 andS2). Single-solvent (ethanol only) drops exhibited various
particle size-dependent structures, including distributed clumps
for large particles (268 ± 51 nm), highly pronounced coﬀeerings for medium particles (110 ± 22 nm), or less pronounceds a drop of liquid evaporates on a solid surface, theparticles within form a deposit. Due to the myriad ofphysical and chemical processes involved during evaporation,
the structure of this deposit can vary greatly, ranging from
simple ring formations like those seen in coﬀee drops,1 to more
complex morphologies such as fractal patterns.2,3 Of these
possible outcomes, a strikingly uniform deposit may be formedby mixing together two cosolvents.4,5 Although common
wherever there are solvent mixtures (e.g., most kitchens, dining
rooms, or pubs), these deposits also show great potential forindustrial applications, as they may provide simpler, faster, andless costly ways to manufacture thin ﬁlm devices compared toconventional methods.6−9 By appropriately tuning design
parameters, co-solvent-based deposits could also be used to
produce conformal monolayer coatings on three-dimensionalsurfaces or extremely ﬁne hole patterns. Here we ascribe theuniform deposits associated with cosolvents to a hybrid selfassembly process. The cosolvent drop segregates during
evaporation, resulting in an elevated concentration of higher
vapor pressure solvent at the drop surface.10 Consequently, theparticles, miscible only in the higher vapor pressure solvent, arecarried toward the drop surface. Once at the drop surface, theparticles self-assemble due to attractive particle−interface
interactions.11,12 Real-time observations during evaporation
reveal the mechanisms of this self-assembly process. The
observed drop proﬁle kinetics agree well with our evaporation
model for cosolvent drops. We also show that ﬁlm uniformity
depends on an optimal initial particle concentration, below
which results in voids and above which exhibits a coﬀee ring
© 2016 American Chemical Society

Received: December 27, 2015
Accepted: March 23, 2016
Published: March 23, 2016
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Figure 1. Self-assembly process. (a) Multiscale schematic of drop at the onset of evaporation including a macroscale depiction of the drop with
nanoparticles uniformly dispersed throughout, a nanoscale representation of a single nanoparticle covered with a self-assembled monolayer of thiol,
and a molecular-scale diagram of the chemical structure of the thiol. (b−d) Representative top view micrographs and corresponding side view
schematics of self-assembly process at (b) an early stage of evaporation (<10% of evaporation time), (c) middle stage of evaporation (50% of
evaporation time), and (d) deposit after evaporation. Arrows drawn at surface of drop represent a simpliﬁed depiction of the evaporative ﬂux. Arrows
drawn inside drop display a cartoon of internal streamlines induced by evaporation and surface tension gradients. Scale bars are 150 μm in length.

The direction of the Marangoni ﬂow can be deduced byconsidering the inﬂuence of gradients in temperature, ethanol
concentration, and nanoparticle concentration on surface
tension gradients. A detailed discussion and relevant calculations are included in Section 1 of the Supporting Information;
however, we include a brief overview here. The Marangoni ﬂow
may be either thermally driven or concentration driven, where
the dominant case dictates the direction of the internal droplet
ﬂow. The mismatch in thermal conductivities between thesubstrate and the liquid indicates a Marangoni ﬂow from the
top of the drop to the bottom along the liquid−air interface andfrom the contact line toward the center of the drop along theliquid−solid interface (opposite of what is depicted in Figure 1)
and a corresponding thermal Marangoni number, MaT = 5.18 ×
103. On the other hand, given the higher evaporation rate ofethanol compared to water, it is expected that there will be anelevated concentration of ethanol at the contact line where theﬂux is the highest. Because ethanol also has a lower surface
tension, this results in a concentration driven surface tension
gradient opposing the thermally driven gradient and a ﬂow
opposing the one brought about by thermal gradients (in thesame direction as that depicted in Figure 1). The magnitude of
the concentration driven Marangoni number associated withthe gradients in ethanol concentration is MaC = 1.53 × 107,
which is 4 orders of magnitude larger than MaT and therefore
dominates the direction of the ﬂow. Moreover, the inﬂuence ofthe nanoparticles can further contribute to this ﬂow in twoouter coﬀee rings with interior fractal-like deposits for small
particles (84 ± 18 nm). We note that fractal patterns wereobserved at the contact line for single-drops containing all
particle sizes (see Figure S1). With the inclusion of a lower
vapor pressure solvent (water in this case), the resulting deposit
structures became strikingly uniform, ranging from thick, in thecases where the contact line becomes depinned early during theevaporation process (see Figure S2), to extremely thin, when
the contact line remains pinned throughout evaporation. This
study focuses on the latter case.
Figure 1 shows the self-assembly process for particle
monolayers via coevaporation dynamics (see also S1). Notethat the areas of assembled particles appear visually bright withslight color diﬀerences due to light scattering from thecurvature of the drop surface. As evident from Figure 1b,
assembly begins at the onset of evaporation when a ring of
nanoparticles nucleates at the contact line. This is expected,
because particles within evaporating drops will tend toward thesurface where evaporation ﬂux is the highest.1,18 Following
nucleation, rather than further accumulation of nanoparticles atthe boundary to form a previously observed coﬀee ring,1 thestructure transitions into a uniform sheet over the surface of thedrop (Figure 1b) as the surface area is reduced and more
nanoparticles are introduced to the interface. Note that the
Marangoni ﬂow within the drop during evaporation recirculates
nanoparticles below the surface until they are introduced to theinterface.
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Figure 2. Evaporation process. (a) Proﬁle images of sessile drops with various particle concentrations at diﬀerent times (beginning, middle, end)
during the evaporation process. Vertical dashed lines are used to highlight that the drops remain pinned (i.e., constant contact radius) throughout
this process. Concentrations are given in grams/liter (g/L). Scale bars are 150 μm in length. (b−e) Kinetics of drop geometry during evaporation,
including (b) contact angle, (c) volume, (d) dimensionless contact angle plotted with respect to time normalized by drop size and initial contact
angle, and (e) dimensionless volume with respect to time normalized by drop size and initial contact angle. Vertically oriented dashed lines mark the
total evaporation time for each experiment. The theory curves were generated based on observed initial conditions for the 0 g/L drop experiment(see Section 2 of SI). For reference, the particles for the deposits in this ﬁgure had a mean size of 268 nm with a standard deviation of 51 nm (seeExperimental Section and Figure S4).

ways. First, the areas of the drop surface that are densely packed
with nanoparticles (initially at the contact line) will exhibit alower surface tension compared to areas that are particle free(see Figure S3), resulting in a surface ﬂow from the edge of thedrop to its center along the liquid−air interface. Second, thenanoparticles can be viewed eﬀectively as surface contaminants,
which can further reduce MaT by as much as 2 orders ofmagnitude,19 thus increasing the relative dominance of theconcentration driven ﬂow.
The monolayer growth at the drop surface continues
throughout the evaporation process until the higher vapor
pressure solvent has completely evaporated and all of theparticles have reached the interface (Figure 1c). During theﬁnal stages of the evaporation process, the contact line of theevaporating ﬂuid depins, allowing for the assembled sheet of
nanoparticles to collapse onto the substrate (Figure 1d). Theseobservations suggest a process qualitatively diﬀerent frompreviously reported methods for producing uniform ﬁlms from
evaporating drops, which solely employ interparticle capillaryinteractions,20 particle−interface interactions,21 or ﬂuid−
particle interactions.19,22 Instead, this assembly process appears
to be a hybrid case wherein particle−ﬂuid interactions carry theparticles to the boundary, followed by particle−interface
interactions that assemble the particles into a sheet at thesurface of the drop. Moreover, we observed sheet fracturing(see S1, 0:27−0:35) in drops with low particle loading andsheet buckling (see S2, 1:18−1:53) in drops with high particle
loading, similar to that seen in evaporating drops containing
latex particles.23 This observed mechanical behavior of the
sheet (i.e., fracturing of sheets in tension associated with drops
with low particle loading and buckling of sheets in compression
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Figure 3. Tuning particle concentration eliminates coﬀee ring. (a) Optical micrographs (left) of deposits left behind (scale bars are 150 μm in
length) with corresponding detail confocal intensity micrographs (right) of the deposit edge (scale bars are 10 μm in length). (b) Corresponding
proﬁles at edge of deposits. Solid lines represent average proﬁle. Lighter shaded regions represent one standard deviation (n = 5) above and below
the average proﬁle. For reference, the particles for the deposits in this ﬁgure had a mean size of 268 nm with a standard deviation of 51 nm (SeeExperimental Section and Figure S4).

concluding the second phase of the hybrid self-assembly
process.
We also examine the eﬀects of particle concentration on theevaporation process (note that nanoparticle concentrations
here are reported per volume of total solution). Figure 2ashows that the drops remained pinned throughout most of theevaporation process, regardless of particle loading (see also
Figure S7). Moreover, Figure 2b shows similar initial contact
angles among all concentrations, indicating that the wetting
behavior is dominated by the interaction between the solvent
mixture and the substrate. Normalizing the data from Figure
2b,c to account for diﬀerences in drop size and shape (seeSection 2.5 of SI for normalization description) enables
comparisons between our cosolvent evaporation model anddrops of various particle loadings. These normalization results
are shown in Figure 2d,e. Note that this normalization isapplicable for evaporating drops with contact angles that areless than 45°,25 which is applicable to our system. Afteraccounting for diﬀerences in drop size and shape, the dynamics
appear to behave in accordance with that predicted from our
mixture evaporation theory, indicating that particle concentration has little eﬀect on the evolution of the drop proﬁleduring evaporation throughout a majority of the drop lifetime.
Moreover, the low model errors (<5%) in the normalized total
evaporation time for low particle concentrations (<15.0 g·L−1
or <2.38 × 1014 particles·L−1 or <0.24 vol %) also indicate agood model ﬁt toward the end of the evaporation process.
However, there is a relatively large discrepancy (≈17% model
error) for the high particle concentration (15.0 g·L−1 or 2.38 ×
1014 particles·L−1 or 0.24 vol %). This indicates that the model
is valid over a reasonable range of concentrations (≤10.7 g·L−1
or ≤1.70 × 1014 particles·L−1 or ≤0.17 vol %), but should be
augmented to account for the eﬀects of the particles at highconcentrations (≥15.0 g·L−1 or ≥2.38 × 1014 particles·L−1 or≥0.24 vol %).

associated with drops with high particle loading) is indicative ofa solid-like nanocrystal.
Evaporation studies were conducted to reveal further the two
mechanisms of this self-assembly process, as summarized in
Figure 2. Figure 2a shows proﬁle images of cosolvent sessile
drops with various particle concentrations throughout theevaporation process. As evident from Figure 2b−e, theevaporation dynamics initially favor that of pure ethanol
despite the fact that the initial concentration of water is muchlarger (initial mole fractions of water and ethanol within thedrop are xw ≈ 0.87 and xe ≈ 0.13, respectively), which supports
the previous experimental observation that the drop rapidly
segregates during evaporation into an outer ethanol shell and awater enriched core.10 The evaporation process can be modeled
as quasi-steady state, where liquid vapor equilibrium applies(see Sections 2.1−2.3 of SI for a more detailed discussion on
how the evaporation process is modeled). In contrast to
previous studies on evaporating drops of ethanol−water
mixtures,10 our model accounts for the azeotropic eﬀect (seeSection 2.4 of SI). As seen in Figure S5, the vapor mole fractionof ethanol just above the drop surface, ye, is signiﬁcantly larger
than the liquid mole fraction of ethanol within the drop (e.g.,
initially for this study xe ≈ 0.13 and ye ≈ 0.49). To maintain thiscondition, an excess of ethanol must be present at the interface.
Moreover, the larger evaporation rate of ethanol during thisprocess (see Figure S6b) ampliﬁes the transport of the ethanol
to the surface. Because it is known that the thiol-capped
nanoparticles form stable colloidal suspensions in ethanol16,17
and are also hydrophobic24 (see also Figure S3), they migrate
to the drop surface via the evaporating ethanol due to ﬂuid−
particle interactions, completing the ﬁrst part of the assembly
process. Once at the interface between the water enriched core
and the ethanol shell, it is expected that the nanoparticles
assemble due to particle−interface interactions,11,12 thereby
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Figure 4. Application to thin ﬁlm optics. (a−b) Qualitative examples demonstrating use of deposits as thin ﬁlm devices. (a) Filters, where the deposit
reduces the amount of light transmitted through (left) a close-up image of Einstein and (right) a close-up image of a Purdue “P”. (b) Mirrors, where
the deposit reﬂects light incident on (left) an image of an array of Einsteins and (right) an image of an array of Purdue “P’s”. Deposits areapproximately 2 mm in diameter. (c) Representative SEM image of deposit morphology for the 84 ± 18 nm samples used in panels a and b (scalebar is 500 nm in length). (d−g) Spectrophotometry data for samples of various particle size including (d) absorbance, (e) transmittance, (f)reﬂectance, and (g) specular reﬂectance normalized by total reﬂectance.

range can be transformed to the number of particles in a drop(n) under the spherical cap approximation (see Section 2.1 of
SI for validation of this approximation) via n = 2cR3(1 +cos(θ0))2(2 + cos(θ0))/d3ρ sin3(θ0); where c is the initial massconcentration of particles, R is the areal contact radius of thedeposit, θ0 is the contact angle of the drop at the onset of
evaporation, d is the particle size, and ρ is the mass density ofthe particle. Substituting in appropriate values for theseexperiments: 7.3 g·L−1 < c < 8.8 g·L−1, R ≈ 1.2 mm, θ0 ≈25°, ρ = 6250 kg·m−3, and d = 268 nm (the particle size used
for the study in Figure 3), results in an optimal number ofparticles in a drop to be between 7.1 × 107 and 8.5 × 107.
Theoretically, a closely packed monolayer of particles from adeposited drop requires noptimal = 4pR2d−2, where p ≈ 0.907 is
the hexagonal packing factor for circles in a plane.26 We assume
hexagonal packing because the time scale for the particles to

Figure 3 summarizes the inﬂuence of initial particle
concentration on the ﬁnal morphology of the deposit. Deposits
with high initial concentrations (15.0 g·L−1 or 2.38 × 1014
particles·L−1) exhibit a dense accumulation of particles at theboundary (i.e., a large coﬀee ring), with a uniform deposit at theinterior of the drop. Note the presence of cracks in the coﬀee
ring of the high concentration deposit, indicative of highparticle loading in dry colloidal deposits.17 As evident from
Figure 3, reducing the initial concentration results in thinner
thickness of the uniform interior of the deposit as well as a size
decrease of the coﬀee ring, with a complete elimination
corresponding to concentrations <8.8 g·L−1 (1.40 × 1014
particles·L−1). Small voids in the deposit at the smallest initialconcentration (7.3 g·L−1 or 1.16 × 1014 particles·L−1) suggestthat the optimal initial concentration that will result in a
uniform monolayer deposit lies between 7.3 and 8.8 g·L−1. This
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Our experiments have revealed that uniform deposits from
evaporating cosolvent drops are created by a hybrid of two selfassembly mechanisms: (1) ﬂuid−particle interactions, where
the nanoparticles are carried to the drop surface by the higher
vapor pressure solvent, followed by (2) particle−interface
interactions once the particles reach the interface of the phasesegregated drop. As observed, this assembly process occurs at
the surface of the drop, indicating independence of thismechanism with respect to substrate. The cosolvent evaporation model we have developed accurately describes the drop
proﬁle kinetics and could be used for future design of cosolvent-based functional inks. The agreement of the size andshape normalized data and model show scalability of thisprocess. Our results also show a simple means for designing
particle concentrations to produce uniform monolayers.
Moreover, we demonstrate the application of cosolvent
formulation to high performance drop-on-demand thin ﬁlm
devices such as mirrors and broad band neutral density ﬁlters.
Given these results, it is likely that high throughput drop
deposition methods (e.g., roll-to-roll inkjet printing) will
receive more attention as a viable means to fabricate thin ﬁlm
devices in the future.

assemble by particle diﬀusion is found to be smaller than thetime scale for the particles to reach the drop surface due toevaporation (see Section 3 of the SI), indicating that the likely
arrangement of the particles at the surface is hexagonal
packing.27,28 Therefore, the theoretically estimated number ofparticles per drop to produce uniform monolayer deposits is
noptimal = 7.3 × 107 (corresponding to an optimal concentrationof coptimal = 7.5 g·L−1), which indeed falls within our observed
range. This demonstrates a process for determining appropriate
particle concentrations for uniform monolayer deposits incosolvent formulations.
On the other hand, these results also indicate that exceeding
the appropriate concentration range will result in the formationof a coﬀee ring at the contact line and an increase in thickness
over the uniform interior of the deposit, which is expected dueto the nature of monolayer growth during evaporation. Once amonolayer is achieved during evaporation, any particles
remaining within the drop will begin to form multilayer
structures as they move to the surface and collide with particles
within the formed monolayer. The likelihood for collisions islargest at the edge of the drop because this is where the radial
velocity is highest,19 resulting in the presence of coﬀee ring
deposits for c > coptimal, as shown in Figure 3. Note theappearance of gray spots in the optical micrographs of Figure 3afor c > coptimal. These gray spots in the optical microscope
images of the deposits are a result of diﬀerences in lightscattering, indicating diﬀerences in local ﬁlm thickness (lower
local thicknesses in the gray areas). We suspect that this is a
result of multiple layers being formed throughout areas ofsurface while the ﬁrst layer is still being formed in others.
Experiments were conducted to demonstrate application ofthis self-assembly process to the fabrication of high performance drop-on-demand thin ﬁlm optical devices, summarized in
Figure 4. Figure 4a,b qualitatively highlights drop-deposited
devices such as ﬁlters and mirrors. The representative scanningelectron micrograph in Figure 4c reveals that these deposits are
uniform and densely packed throughout. Figure 4d−g shows
the measured optical results for self-assembled deposits ofvarious nanoparticle size. The larger particles will produce
thicker samples, resulting in an increased absorbance, as seen in
Figure 4d. This gives rise to the reverse eﬀect for thetransmittance (see Figure 4e). The absorbance and transmittance spectra also ﬂatten with increased particle size. Thismay be due to the increased size heterogeneity with respect toparticle size (see Figure S4), allowing for cancellation ofresonance peaks associated with diﬀerent sized particles within
the sample. Most notable of this eﬀect is the ﬂat transmission
response (13% ± 2%) associated with the largest particle (d =268 nm) monolayer deposits over the entire sampled range(300−820 nm), which demonstrates an application of these
deposits as a broadband neutral density ﬁlter. As shown in
Figure 4f,g, the samples are also inherently reﬂective given their
metallic composition. The decrease in particle size gives rise tosmoother deposits. As the deposits become smoother, their
total and specular reﬂectances increase, resulting in the trend
we see here. Regarding application, as shown in Figure 4g, thehigh specular reﬂectance response (>80%) associated with thesmall particle (d = 84 nm) deposits shows promise for use as a
mirror, especially for wavelengths in the visible−near-infrared
range. These results are remarkable considering the ﬂexing andstretching experienced by the samples during optical measurement preparation (see Experimental Section).

■

EXPERIMENTAL SECTION

Preparation of EGaInNP Dispersions. EGaInNP dispersions
were prepared by an ultrasonication method similar to previous
works.16,17 All samples were prepared in 3 dram bottles (Kimble
Chase). Before ultrasonication the vials were thoroughly washed with
Liquinox detergent (Alconox), followed by a rinse with distilled water,
followed by a rinse with ethanol to remove surface impurities. Once
clean, a nominal mass of 1 g of eutectic Gallium−Indium alloy(EGaIn) (Sigma-Aldrich) was added to the vial via syringe. Masses
were measured using a Ohaus Pioneer balance with 0.1 mg resolution.
This was followed by the addition of 120 μL (Labnet BioPette, 1000
μL size) of 100 mM solution of 3-mercapto-N-nonylpropionamide(1ATC9, Sigma-Aldrich). An additional 3880 μL of pure ethanol wasadded to achieve a ﬁnal thiol concentration of 3 mM and ﬁnal liquidvolume of 4 mL. Once the sample was prepared, the liquid metal was
dispersed into the solvent using a Qsonica Q700 probe ultrasonicatorﬁtted with a 1/16 in microtip. The vial with EGaIn and solvent was
contained in a water bath to prevent overheating and evaporation of
the sample. The sample was sonicated for 1 h at an amplitude of 30%.
After sonication, samples were either further processed via
centrifugation or placed in a −35 °C freezer for later processing.
Diﬀerential centrifugation was used to separate particles of various
sizes. The process is as follows. Sonicated samples were distributed
evenly into 1.5 mL polypropylene centrifuge tubes, lightly bathsonicated (Branson 1800) in isopropyl alcohol (Macron Fine
Chemicals) for approximately 20 s, and placed in a refrigerator atapproximately 10 °C for several minutes to allow the samples to reachbelow the melting point of the bulk alloy. The samples were thenrinsed of any free thiol by centrifugation (RevSpin RS-200). Namely,
the samples were spun at 10,000 rotations per minute (RPM) (5529
relative centrifugal force (RCF)) for 10 min. The resulting aliquots
were discarded and the pellets were resuspended via light sonication inethanol. This process was repeated three times to ensure there was nounbound thiol in the samples. Next, the resuspended pellets were spunat 2,000 rpm (220.5 RCF) for 10 min. The aliquots from this step
were poured into centrifuge tubes to obtain smaller particles, whereas
the pellets were resuspended via light sonication in fresh ethanol. This
process was repeated three times. This procedure was further applied
to the aliquots to obtain particles at 4,000, 6,000, and 8,000 rpm (884,1990, and 3537.5, RCF, respectively) in series. Samples from eachdiﬀerential centrifugation condition were drop deposited on clean Si
for scanning electron microscope (SEM)-based particles sizing (seeFigure S4). Ethanol/water mixtures were made by adding a known
volume (Lagnet BioPette, 20 μL) of ethanol to a pellet of dried

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surface, and a surface pressure sensor, which controls the barriers.30
Monolayers of EGaINPs were prepared using a spreading ethanolic
solution with a concentration of 50 mg·mL−1. The sample was spreadcarefully using a positive displacement pipette at a rate of ≈0.6 μL·s−1.
After the solvent evaporated, the hydrophobic dodecanethiol(1ATC9)-stabilized EGaInNPs remained on the water surface,
appearing as a shiny silver color (see Figure S3), and were thencompressed by moving the barriers at a speed of 10 mm·min−1. The
surface pressure isotherm was recorded throughout the compression.
The temperature of the double deionized water (18 MΩ·cm in thetrough) was about 20 °C.

particles, followed by light bath sonication for resuspening, followed by
the addition of the appropriate volume (Lagnet BioPette, 20 μL) of
distilled water to reach the required mixture ratio (for this study an
ethanol mass fraction of ≈28% was used).
Preparation of Substrates. The polydimethylsiloxane (PDMS,
Sylgard 184, Dow Corning) samples were prepared as follows. The
polymer is provided as a two-part system consisting of a base and acuring agent. The two liquids are mixed in a 10:1 mass ratio. The twoparts were mixed by hand and placed in THINKY ARE 310 orbital
mixer for further mixing and degassing. Once mixed, the liquid
polymer was cast onto 2 in. × 3 in. borosilicate glass slides using aSpecialty Coating Systems Spin-coat G3-8 spin coater at 200 rev·min−1
for 60 s. Coated slides were cured in an incubator at 60 °C for at least
4 h. Upon curing, the polymer ﬁlm was cut into 0.25 in squares, andcleaned by sonication in acetone, followed by rising with acetone,
isopropyl alcohol, ethanol, and water. The rinsed samples were thendried with compressed air. PDMS squares were removed from thediced sample and placed on a clean glass carrier slides for plasma
treatment. Slides were treated with oxygen plasma (Plasma Etch Venus
25 plasma etcher) for 5 min at 50 W with an oxygen ﬂow rate of 10
CFM·min−1. Treated surfaces were used within 24 h to maintainconsistent drop wetting and contact line pinning behavior (see FigureS8). The Si substrates employed for drop-depositing monolayers (seeFigure S9) were prepared using a procedure described elsewhere.29
Once received, they were cleaned via bath sonication in acetone,
isopropyl alcohol, then ethanol, for 5 min each, drying with air after
each sonication treatment.
Drop Evaporation Experiments. All drop evaporation experiments were conducted at room conditions with a measured
temperature (Fluke 87 V with K-type thermocouple) of 23 ± 2 °C
and a measured relative humidity (Kele HS-2000D) of 21% ± 2%.
Typical experiments for conducting drop evaporation were conducted
as follows. Sessile drops were formed by gently placing small volumesof colloidal dispersion (see previous Experimental Subsections) via
pipette (Lagnet BioPette, 20 μL) onto stationary substrates located ona microscope stage (Zeta 20 with custom tilt attachment). The
substrates employed for observing the evaporation process were
plasma treated PDMS (see Preparation of Substrates). Upon
deposition of drops, top and side views were ﬁlmed from the optical
microscope with a screen capture software (Camtasia 8). Using an inhouse MATLAB image analysis script, the geometry of the proﬁleswere analyzed throughout the evaporation process until the proﬁlebecame too small for the script to detect (between 4° and 9°,
depending on initial drop size, objective, and focusing conditions).
Drop deposits were also made on Si substrates (representative results
shown in Figure S9).
Deposit and Particle Characterization. Samples for particle size
characterization were prepared by depositing small volumes of eachdiﬀerential centrifugation condition via micropipette (Lagnet BioPette,
20 μL) onto clean Si substrates. SEM images of these samples werethen obtained (Philips XL-40 FEI). All images were analyzed using
previously reported procedure17 (results from this process can befound in Figure S4). Topography measurements of the self-assembled
deposits on PDMS substrates were obtained using a confocal
microscope (LEXT 3000) with a 100× microscope objective. The
PDMS squares were then mechanically peeled from the carrier glass
slide and placed onto smaller glass slides for spectrophotometry
measurements (PerkinElmer Lambda 950, light scan from 300 to 820
nm, with a resolution of 0.5 nm and a 5 s dwell time). Following
topography and spectrophotometry measurements, the self-assembled
deposits on PDMS were prepared for SEM (FEI Nova nanoSEM
FESEM) imaging (representative result shown in Figure 4c) via
platinum sputtering (Cressington 208 HR, 40 mA, 0.08 mbar for 60 s).
Self-assembled drop deposits were also placed on Si substrates using
the same method as the drop evaporation experiments and imaged via
SEM (FEI Quanta 2D FEG Dual-beam SEM) (result shown in Figure
S9).
Langmuir Trough Experiments. The instrument used in thiswork is a Kibron microtrough with a pair of automated movablebarriers, which compresses the nanoparticles that are spread on a water■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.5b12687.
Summary table of model parameters; discussions on
Marangoni ﬂow direction, theory and model for
cosolvent evaporation (including model assumptions,
thermal and convective eﬀects, mixture considerations,and normalization of parameters), and diﬀusive timescale versus hydrodynamic time scale; ﬁgures showing
SEM image of fractal patterns, optical images of deposit
outcomes for varied solvent formulations and substrate
treatments, Langmuir trough experimental results,
process for obtaining particle sizes, vapor−liquid
equilibrium data for ethanol−water mixtures, model
predictions for drop mass and composition dynamics,
observed contact radius during evaporation, postplasma
time eﬀects on drop wetting and contact line pinning,and demonstration of monolayer via the hybrid selfassembly process (PDF).
Movie S1, showing the hybrid self-assembly process(MOV)
Movie S2, showing the shell buckling for drops with highconcentrations of nanoparticles (MP4)■

AUTHOR INFORMATION

Corresponding Authors*J. W. Boley. E-mail: jwboley@seas.harvard.edu.
*R. K. Kramer. E-mail: rebeccakramer@purdue.edu.
Present Address§

John A. Paulson School of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138, UnitedStatesNotes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS
This work was partially supported by an Early Career Faculty
grant from NASA’s Space Technology Research Grants
Program (NNX14A052G). E.L.W. is supported under the
National Science Foundation Graduate Research Fellowship
program (DGE-1333468). The authors thank Professor
Vladimir Shalaev for providing access to the spectrophotometerequipment and Dr. Alexei Lagoutchev for training on the
spectrophotometer.

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NOTE ADDED AFTER ASAP PUBLICATIONThis paper was published on the Web on April 1, 2016, with anincomplete Supporting Information ﬁle. The corrected version
was reposted on April 26, 2016. Two Supporting Information
movie ﬁles were added, and the corrected version was reposted
on July 26, 2016.

34178

DOI: 10.1021/acsami.5b12687
ACS Appl. Mater. Interfaces 2016, 8, 34171−34178