Adhesion of Morphologically Distinct Crystals to and Selective

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Adhesion of Morphologically Distinct Crystals to and Selective Release from Elastomeric Surfaces Mark A. Rose, Jay M. Taylor, and Stephen A. Morin Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02575 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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Chemistry of Materials

Adhesion of Morphologically Distinct Crystals to and Selective Release from Elastomeric Surfaces Mark A. Rose,† Jay M. Taylor,† Stephen A. Morin*†‡ †

Dept. of Chemistry, University of Nebraska-Lincoln, Hamilton Hall, Lincoln, NE 68588.

‡

Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE 68588.

ABSTRACT: This manuscript describes the adhesion characteristics of micron-scale crystals to chemically-functionalized, soft elastomeric supports throughout repeated cycles of tensile stress. The ability to tune the characteristics of hard/soft interfaces (e.g., those formed between silicones, such as polydimethylsiloxane, and semiconductors, such as ZnO) is fundamentally important to understanding processes in biomineralization and crucial to technologies such as stretchable electronics and self-cleaning, anti-fouling surfaces. This study systematically examined the effect of surface chemistry, crystal morphology, the ambient environment, and the number of stress events on adhesion by monitoring the displacement and delamination of crystals from the support polymer. The crystals studied remained surprisingly well adhered, even in wet environments. These understandings were applied to the design and synthesis of surfaces that, through a process of mechanically-activated, selective delamination, could generate 2D patterns of crystals from random deposits or isolate specific morphologies from mixtures. The concepts presented here could be applied to material assembly or to the separation of micron-scale particles useful to, for example, the analytical/forensic sciences.

1. Introduction The interface between hard and soft materials is critical to a range of technologies (e.g., stretchable electronics,1 biosensors,2 etc.) and is fundamentally important to the processes of biomineralization3 and the design of durable composite materials.4 These interfaces are characterized by a stark mechanical mismatch between hard materials (usually the functional elements) and soft materials (usually the substrates) that often results in delamination or deformation (i.e., buckling, fracture, etc.) of the hard material when the interface is stressed. Several strategies (e.g., adhesive and graded interfaces5-6) have been explored to avoid such failure mechanisms (i.e., delamination and fracture) that can occur when these systems are stressed; however, investigations that examine the adhesion of hard, crystalline materials of various morphologies to soft support substrates in the absence of adhesives (that is, the characteristics of self-adhesion), through several cycles of stress, remains limited. In this work we used microcontact printing to place crystals of distinct morphologies and sizes, high and low surface-contact areas, and roughly the same surface chemistry onto chemically functionalized elastomeric substrates. We then tracked (by mapping particle trajectories) the displacement and/or delamination of these crystals as the substrate was cycled between two different values of strain (specifically focusing on controlled, largemagnitudes of strain, ε = 1.0). This ensemble study, which included hundreds of cycles of stress in both dry and wet environments, allowed us to describe adhesion of particles in terms of displacement and delamination trends quantified from a diverse set of conditions. In the majori-

ty of the conditions studied, even those involving wet environments, the crystalline materials remained surprisingly well-adhered to the surface exhibiting little displacement despite hundreds of cycles of stress to the underlying substrate. The results we describe offer perspective on: (i) how the interactions at soft/hard interfaces can be tuned using surface chemistry (in the absence of adhesives, etc.), and (ii) how hard materials move and delaminate from soft surfaces of different chemistries. Practically, this work provides a convenient method for the rational manipulation of a large number of crystals potentially useful to structure-property relationships and/or assembly, and routes toward the development of mechanicallyactivated, “self-cleaning” materials that can expel microscale particles (i.e., crystals or dust) upon mechanical stimulation. In demonstrations of these capabilities, we assembled 2D patterns of crystals and separated mixtures of crystals using selective, mechanically-induced delamination. The mechanical mismatch of soft/hard interfaces has been extensively explored, and includes pioneering examples where thin films of precious metals attached to soft elastomeric supports were shown to spontaneously assemble into wrinkled microstructures upon the introduction of controlled compressive stresses.7-9 Mechanistic understanding of the formation of these wrinkled microstructures has been developed through characterization of a model system consisting of silicon ribbons covalently attached to siloxane-based rubbers (i.e., polydimethylsiloxane, PDMS).10-11 Additional studies have explored the use of adhesives (e.g., conductive pastes),12 which can

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eliminate the dependence on silicone-based polymer substrates (which benefit from plasma-based bonding techniques) and the need for adhesive metal under-layers (e.g., titanium).7-8 It has been shown that in some cases these adhesion methods could influence the materials properties (mechanical, electronic, optical, etc.).13 Another method to accommodate high stress without fracture and delamination, while still minimizing the negative effects of adhesives, relies on films that have been engineered to limit the effects of mechanical mismatch through the fabrication of hard/soft interfaces with a gradient in stiffness, thus distributing stress throughout the film as opposed to focusing it at the hard/soft interface.5-6

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rods.26 Furthermore, the technological applications for ZnO (and the related ZHS) are numerous and include, for example, optoelectronics,27-28 sensing,29 and piezoelectrics.30

Apart from the use of adhesives or specialized films, self-adhesion has also been explored as it could offer alternatives which are not limited to materials with specific chemicals properties/compatibility necessitated by bonding/adhesives. Previously, we demonstrated that, using self-adhesion and compressive stress, thin films of inorganic materials synthesized on silicones formed wrinkled microstructures that remained adhered to the surface through hundreds of cycles of stress.14 Others have shown that the hard/soft interface can be tuned using surface modification techniques (e.g., self-assembled monolayers, SAMs, or polymer grafting),15-16 which, by providing methods to control precisely the surface functional groups present, enabled one to strengthen (or weaken) the attraction between two materials.17 In a related strategy, we demonstrated the ability to synthesize chemical patterns on elastomeric surfaces and that these patterns were stable through hundreds of stress cycles.18 Self-adhesion of hard materials on soft surfaces tuned using such surface derivatization methods was able to minimize delamination or fracture so long as stress was small (ε ≤ 0.25) and/or the surface area of the hard/soft interface was minimized.19-22 This self-adhesion has been explained by a “pinning” effect—the stress placed on the hard “islands” was minimized by the soft support, which was pinned, locally at the hard/soft interface, redistributing stress around the island.19 2. Experimental Design We selected ZnO as the hard crystalline material for this investigation because it features a wurtzite structure with Zn2+ tetrahedrally coordinated by O2- in an ABAB stacking motif which gives rise to non-polar and polar facets. We used chemical bath deposition for the growth of ZnO because it allowed for easy access to different morphologies of ZnO through minor modifications (i.e., addition of citrate) to the precursor solutions.23-25 Specifically, we synthesized 1D rods and 2D discs of ZnO (Figure 1D, G, and see supporting information, S.I. text) which featured similar surface area and surface chemistry. For a third morphology, we chose zinc hydroxysulfate (ZHS) plates (Fig. 1J) because they presented a similar surface chemistry on the dominate (0001) facet as that of the (0001) facet on ZnO, while having a 2D morphology of relatively high surface area compared to the discs or

Figure 1. A) Schematic of the microcontact printing process. B-D) Optical micrographs (B, C) and scanning electron micrograph (D) of ZnO discs (scale bars are 50 µm, 20 µm, and 5 µm respectively). E-G) Optical micrographs (E, F) and scanning electron micrograph (G) of ZnO rods (scale bars are 50 µm, 20 µm, and 1 µm respectively) H-J) Optical micrographs (H, I) and scanning electron micrograph (J) of ZHS plates (scale bars are 50 µm, 20 µm, and 10 µm respectively). The annotated boxes indicate “observation wells” formed by the microcontact stamp.

We performed ATR-FTIR spectroscopy on the respective morphologies to ensure that the capping ligands (e.g., polyamines, citrate, etc.) present during synthesis were, if persistent through washing steps, similar in all cases (Fig. S1). These studies confirmed that we could remove the majority of these ligands using a standard washing proce-

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dure and that any ligands that persisted were similar for all morphologies (Fig. S1 and S.I. text). Further, we measured the zeta potential for each of the different crystal morphologies to determine if differences in electrostatics might dominate adhesion (Table S1). We observed that the zeta potentials of all the crystals were negative (ranging from −24.42 to −30.91 mV with the ZnO crystals being slightly more negative on average than the ZHS) and thus expected similar electrostatic contributions for all morphologies. We confirmed the crystals to be ZnO or ZHS using powder X-ray diffraction (Fig. S2). We chose PDMS as the elastomeric substrate due to the mature body of literature reporting on its chemistry and mechanics.31-32 We synthesized PDMS surfaces with three distinct chemistries: (i) the native methylated surface (PDMS-CH3), (ii) a hydroxylated surface (PDMS-OH), and (iii) a surface functionalized with a perfluorinated alkane (PDMS-F). We expected different crystal adhesion properties on each of the surfaces based upon varying degrees of favorable interactions at the hard/soft interface. Specifically, polarity (e.g., dipole-dipole interactions) and van der Waals (VW) forces should dominate interactions between the polymer substrates and the crystals. We thus expected that the attractive forces important to adhesion should be strongest for PDMS-OH and weakest for the PDMS-F, where the former provides both dipole-dipole and VW interactions while the latter only supports weak VW interactions.33 The PDMS-CH3 provided an intermediate case where dipole-dipole interactions were minimized but reasonably strong VW forces remained.33 The relative surface energies, as measured via contact angle of water (Table S2), followed the suggested trend. Additionally, we estimated the expected Hamaker constants (A132), which quantify the attractive forces expected via VW interactions, for ZnO relative to PDMS, silica, and Teflon (the surfaces of silica and Teflon, which are widely reported in literature, are expected to be similar to PDMSOH and PDMS-F surfaces respectively; Table S3 and S.I. text).34-36 We used crystal displacement and delamination through stress-cycling of the supports as a metric to investigate the adhesive properties expected. We focused on uniaxial (as opposed to biaxial) tensile stress because it provided a convenient method to control strain in the polymer supports that was prevalent in commercially available electromechanical testing systems and easily adapted to home-built devices (Fig. S3). We adopted an approach that used microcontact printing37 (Fig. 1A) to place crystals onto the surface of the elastomer substrates and optical microscopy to follow their movement through stress cycles because these methods allowed for the rapid generation and analysis of large-area patterns of crystals. These patterns provided convenient “observation wells,” which simplified the process of particle tracking using image processing tools (Fig. 1 B-C, E-F, H-I, see S.I. text). Additionally, we could control the density of the crystals transferred onto the elastomeric sur-

face. This approach, also provided a convenient way to tune the chemistry of the interface—stamped discs and plates tended to rest on the polar (0001) facets, whereas the rods tended to rest on the non-polar (11̅00) facets. 3. Results & Discussion To investigate self-adhesion on PDMS, we systematically varied surface chemistry, environment, and crystal morphology through a series of experiments that consisted of 18 trials. Each trial included the observation and measurement of at least 10 individual crystals at 4 points along the stress-cycling process (ε = 1.0, and 600 total cycles were used), giving a total of 720 individual measurements. We generated trajectory maps of the crystals (which track displacement in the XY plane, Fig. S4 and S5) and used them to calculate the magnitude of displacement for each trial. We displayed the displacement data in two different ways: (i) absolute displacement from the origin, which quantifies how far, on average, the crystals were displaced from their starting locations, but that gave no indication of the distance the crystals displaced during each segment (0-100, 100-300, or 300-600 cycles) of the 600 total stress cycles (Fig. 2A-C, 3A-C, and S.I. text), and (ii) displacement per step, which quantified the average displacement of the crystals observed after each segment of the 600 total stress cycles (Fig. 2D, 3D, Table S4, Table S5, and S.I. text). The later was an indicator of the total displacement observed along the X and Y axes over the course of the 600 cycles (Fig. 2D, 3D), whereas the former indicated how far, on average, a crystal would be from its starting position following 600 cycles (Fig. 2AC, 3A-C). By comparing the average displacement of crystals from the origin (along the X and Y axes) to a “neutral” line (black line, Fig. 2A-C, 3A-C), which corresponded to equal displacement along X and Y axes, we determined if crystal displacement was biased along a particular axis (e.g., the axis of stress). If bias was present the absolute magnitude of displacement would tend to drift away from the neutral displacement line with repeated cycles. As a final metric of displacement, we calculated the average total displacement over the entire 600 cycles for each condition studied, (Fig. 2E, 3E, Table S4, Table S5, and S.I. text). This metric was convenient for comparing the extent of displacement observed for the different morphologies on different surfaces and/or in different environmental conditions. We further characterized differences in adhesion under different conditions by measuring the delamination of crystals from the substrates, where, for this study, we explicitly defined delamination as the complete removal of a crystal from the surface of the substrate. We quantified delamination, by measuring the change in the total top-down area of all crystals within an observation zone over the course of the 600 stresscycles—a decrease in total area corresponded to the loss of material to the environment (Fig. 4).

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error that could have resulted from the uncertainty in the starting positions of the fragments. These metrics, including displacement from the origin (Fig. 2A-C, 3A-C), displacement per step (Fig. 2D, 3D), and delamination (Fig. 4), are discussed in detail in the following sections. To aid in these discussions and identify significant trends, statistical analysis was performed across the entire data set (Fig. S7).

Figure 2. Crystal displacement when the support substrates were stress-cycled (ε = 1.0 along X) in air. A-C) Each trace represents the average displacement of the observed crystals along the X and Y axes relative to their starting position (0, 0) when the substrate was stress-cycled. The four data points for each line were measure after 0, 100, 300, and 600 stress-cycles (N = 10 crystals for each point). The black line indicates equal amounts of displacement along X and Y. D) Average magnitude of displacement from the previous location in both the X and Y components over the course of 600 stress cycles with images analyzed after 100, 300, and 600 cycles (N = 10 crystals for each point). E) Average magnitude of total displacement after 600 total cycles (N = 10 crystals per point).

We found that displacement of crystals on the substrates proceeded via fragmentation, rotation, and/or translation (“sliding”) of the crystals along the surface (or combinations of all of these motions, Fig. S6). The representative crystals used in generating the data set we present did not include crystals which fragmented during the stress-cycling process. In doing so we avoided potential

Figure 3. Crystal displacement when the support substrates were stress-cycled (ε = 1.0 along X) in water. A-C) Each trace represents the average displacement of the observed crystals along the X and Y axes relative to their starting position (0, 0) when the substrate was stress-cycled. The four data points for each line were measure after 0, 100, 300, and 600 stress-cycles (N = 10 crystals for each point). The black line indicates equal amounts of displacement along X and Y. D) Average magnitude of displacement from the previous location in both the X and Y components over the course of 600 stress cycles with images analyzed after 100, 300, and 600 cycles (N = 10 crystals per point). E) Average magnitude of

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total displacement after 600 total cycles (N = 10 crystals per point).

(Fig. 2A-C). This equal displacement in the X and Y directions was further confirmed by the average displacement per step, which was within standard deviation for the X and Y axes for every case studied (Fig. 2D). By finding the change in total area, we determined that delamination for discs and plates was most prevalent on PDMS-F, while delamination of rods was observed to be statistically indistinguishable at 95% confidence on all surfaces (Fig. 4A, S7). We noticed an interesting artifact for plates—this morphology “grew” in area seemingly indicating more plates than were present initially (Fig. 4A). This growth in crystal area was due to the thin, 2D morphology of the plates which made it common for them to stack on top of one another during deposition. Upon the introduction of stress, stacks of plates could “unstack” giving a positive change in area that could be mistaken as more crystals (Fig. S8). This artifact was particularly apparent on the PDMS-CH3, but may also artificially lower the estimated amount of delamination for plates on all surfaces (Fig. 4).

Figure 4. Delamination as quantified by percent change in the total crystal area. The support substrates were stretched (ε = 1.0) in A) air, B) water, and C) 5% (v/v %) Tween 20. The legend given in panel A is shared by all bar graphs (N ≥ 20 crystals per point).

Surface Chemistry. We first explored the effect of surface chemistry on crystal adhesion under ambient conditions (in air) by comparing the behavior of discs, rods, and plates on the different surfaces. We found that the total magnitude of displacement of discs and rods was statistically indifferent, at 95% confidence, for all surface chemistries (Fig. 2E, Fig. S7, and Table S4), with the exception of discs on PDMS-CH3 versus rods on PDMS-OH which were statistically indistinguishable at 90% confidence. Conversely, we observed that plates on PDMS-F surfaces experienced a 500% increase in displacement when compared to plates on either PDMS-CH3 or PDMSOH, which were statistically indifferent (Fig. 2E, S7, and Table S4). When we examined the possibility of a directional bias on the different surfaces, we found, in all cases, that the magnitude of displacement tracked well with the neutrality line or crossed it (sometimes more than once) indicating no preference for displacement along X or Y

Our observations, in terms of displacement and delamination (metrics used to describe adhesion) for discs and plates, roughly mirrored surface energy. Furthermore, the overall trend of this data tracks with the magnitude of the calculated Hamaker constants (Table S3 and S.I. text). Specifically, the smallest Hamaker constant was for PDMS-F (where displacement and/or delamination was maximal) and the largest Hamaker constant was for PDMS-OH (where displacement and/or delamination was minimal) (Table S3). Interestingly, we noted that the displacement and delamination of rods was statistically indifferent across all surfaces (Fig. S7). We believe this observation, while it could be partially explained by the surface chemistry of the “waxy” (11̅00) facet that the rods tended to rest on, was mainly because of the anisotropic morphology of the rods which resulted in a local force balance unlike the other morphologies (as we discuss further in the following section).38-39 We were surprised that the crystals adhered to PDMSOH equally as well as to PDMS-CH3 (statistically, the magnitudes of displacement and delamination were indistinguishable for all crystal morphologies, Fig. S7). We expected that the hydroxyl groups would have stronger interactions with the polar faces of the crystals and that this interaction would strengthen their adhesion to these surfaces more so than to PDMS-CH3 or PDMS-F. This hypothesis was supported by trends in the calculated Hamaker constants (Table S3), but did not agree with our data: though PDMS-F had the weakest adhesion, PDMSCH3 and PDMS-OH were statistically indifferent (Fig. 2, 4A, and S7). We believe one possible explanation for this inconsistency was that the –OH groups on PDMS-OH are not stable in air, especially through stress cycling, and so over the course of an experiment involving PDMS-OH, the surface would become increasingly similar to PDMSCH3.18,40-41

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Morphology. We further examined the adhesion of different crystal morphologies, where plates, discs, and rods made it possible to investigate both the surface area and/or shape of the crystal/PDMS interface. We first considered the effect of interface geometry on adhesion by tracking the displacement of morphologies with roughly the same interfacial contact area but different contact geometries: ZnO rods (rectangular, anisotropic crystal/surface interfaces with an area of 6 ± 2 µm2) versus those of ZnO discs (hexagonal, isotropic crystal/surface interfaces with an area of 8 ± 2 µm2) (Fig. 1D, G). As noted previously, we observed that the magnitude of displacement of ZnO rods compared to the ZnO discs was statistically the same at 95% confidence on all PDMS surfaces (Fig. 2E, Fig. S7, and Table S4). In contrast, we observed that discs had a much greater (400%) instance of delamination on PDMS-F when compared to the rods, while delamination on PDMS-CH3 and PDMS-OH was statistically indifferent at 95% confidence (Fig. 4A and Fig. S7). We attributed the unexpected variation in adhesion for these morphologies on PDMS-F to differences in their shape which led to differences in the local force balance when the substrate was stretched. The introduction of tensile stress creates a shear force on 1D rods that, because of their anisotropic geometry, acts to align rods with the axis of stress (a process observed in other 1D systems).38-39 These shear forces also act on discs, but, owed to the isotropic geometry of these crystals, application of tensile stress (regardless of the axis) to the substrate does not lead to a force imbalance and minimal rotation was expected. We monitored rotation, as it provided a mechanism for accommodating stress that was different for these morphologies, throughout the cycle of stretching/relaxing of the substrate. We observed, as expected, that rotation depended strongly upon morphology of the crystals: the rods tended to align with the axis of stress due to shear forces (Fig. 5A-C, Video S1), whereas discs showed very little rotation (Fig. 5D-F, Video S2). When the long axis of a rod was aligned with (or orthogonal to) the axis of stress, this shear force was balanced and no rotation was observed (Video S1).38 The result was that rods, owed to their anisotropy, allowed for the redistribution of stress (through rotation) more readily than the isotropic discs, and thus delaminated to a lesser extent.

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Figure 5. Optical micrographs of crystals through one stress cycle in air. A, D, G) Crystals before introduction of stress (ε = 0.0). B, E, H) Crystals when a tensile stress (ε = 1.0) was applied to the substrate. C, F, I) Crystals following release of stress (ε = 0.0). A-C) Optical micrographs and annotated traces that illustrate the reversible rotation of rods toward the axis of stress. D-F) Optical micrographs and annotated traces illustrate the minimal amount of rotation of discs. G-I) Optical micrographs with annotated white box indicating the area of a ZHS plate that partially delaminated as evidenced by the change in the interference pattern. All scale bars are 5 µm.

Next, we investigated how differences in interfacial area affected adhesion by comparing large hexagonal plates of ZHS (crystal/PDMS interfacial area of 600 ± 200 µm2) to the smaller ZnO discs (crystal/PDMS interfacial area of 8 ± 2 µm2) (Fig. 1D,J). We expected that the larger surface area of the plates would increase the area of “pinned” polymer chains thus increasing the stress at the crystal/PDMS interface when the polymer was stretched. As a result of this increased interfacial stress, we expected that the plates would experience a greater frequency of delamination and fragmentation and that larger magnitudes of displacement would be observed. Plate displacement followed a “release-stick” mechanism which involved the full or partial delamination of a plate, temporarily relieving interfacial stress associated with stretching the polymer support, followed by “re-pinning” of the plate to the surface when the polymer was relaxed. We note that if a plate re-pined to the surface we did not consider this a delamination event as the plate was not removed completely from the surface. We observed this process of partial delamination and subsequent re-pinning of plates by tracking the changes in the thin-film interference over the course of one stress cycle (Fig. 5G-I and Video S3). Plates were more likely to follow this partial delamination and re-pinning process because they are thin (50-100 nm in thickness)26 and therefore relatively flexible. As expected, upon stress-cycling substrates decorated with plates or discs, we observed the total magnitude of displacement for plates was at least 100% greater than that observed for the disc morphology (Fig. 2E and Table S4). We measured delamination of plates on the PDMS-F to be at least 250% greater than discs which was also consistent with our hypothesis; however, surprisingly, the delamination of plates on PDMS-CH3 was actually lower than discs (Fig. 4A). The tendency of plates to unstack and artificially add to the observed area of the crystal/PDMS interface during stress-cycling (Fig. S8) suggested the results of this comparison could be an artifact; nevertheless, we are confident that delamination was minimal (to virtually absent) for plates on PDMS-CH3. Delamination of both morphologies on PDMS-OH was statistically indistinguishable at 95% confidence (Fig. 4A, S7) and generally delamination on either PDMS-CH3 or PDMS-OH (regardless of morphology) was minimal (less than 2.5%); again, we believe this observation can be at-

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tributed to the instability of hydroxyl groups on the surface of PDMS-OH in air.18,40-41 Environmental Conditions. Finally, we studied how modification of the environmental conditions could modify the adhesion characteristics. To do this we compared the adhesion characteristics for systems that we stresscycled in air versus those that we stress-cycled in water. We found, in general, that the crystals, regardless of morphology, experienced greater magnitudes of displacement in water on all surfaces when compared to those cycled in air (Fig. 3, Fig. S7, and Table S5). The only exception was ZHS plates on PDMS-F surfaces, which were not statistically different in water versus air (Fig. 3E, Fig. S7, and Table S5). This result, generally, agrees with the calculated Hamaker constants, which were consistently an order of magnitude smaller in water versus air, indicating smaller van der Waals attractive forces (Table S3). We found that both the plates and discs experienced a significant decrease (>5%) in delamination on PDMS-F when stress-cycled in water versus air. Interestingly, for the plate morphology this decrease in delamination on PDMS-F was accompanied by a significant increase (>5%) in delamination on PDMS-OH when samples cycled in water were compared to samples cycled in air (Fig. 4A-B). That is, changing from a dry to a wet environment completely flipped the delamination behavior of plates on PDMS-OH versus PDMS-F. We were surprised that the crystals adhered to nonpolar (i.e., PDMS-CH3 and PDMS-F) surfaces as well as they did under wet conditions, especially considering buffered and/or electrolyte rich solutions were not used to stabilize ZnO/ZHS against dissolution. We expected, based on the favorable interactions of the polar facets of the crystals with the surrounding aqueous environment, that the weaker interactions with the relatively non-polar PDMS surface would be easily overcome leading to significant enhancement in displacement and delamination. Our reasoning for why crystal delamination in wet environments was not as prevalent on non-polar surfaces (and more so on polar surfaces) relates to the energy associated with the creation of a solvation layer on the PDMS (to replace the crystal/substrate interface). It was not energetically favorable to solvate a non-polar surface with water, so the crystal/substrate interface was stabilized; it was more energetically favorable to solvate the PDMS-OH and so the crystal/substrate interface was destabilized. Further, in solution, we expected the different PDMS surfaces to have varying surface charges where PDMS-OH would be the most negative surface,42 PDMS-F would also be negative, but to a lesser extent,43 and PDMS-CH3 would be a relatively neutral surface. We expected that the negatively charged surfaces would lead to the formation of electrical double layers (EDL) that would contribute to repulsive forces between these surfaces and the studied crystals (Fig. 3E and 4B), which had negative zeta potentials in aqueous solutions (Table S1 and S.I. text), thus weakening adhesion. Our observations in low ionic

strength solutions were consistent with the potential role of the EDL, as PDMS-OH surfaces showed increased delamination in nearly all systems studied (the exception being ZnO rods, Fig. 4B). To further confirm the role of the EDL, we performed a set of experiments at increasing ionic strength as it was expected, from previous reports,35,44-45 that this would increase short-range repulsive forces via compression of the EDL. In these studies we found that, as ionic strength increased, the increase in the amount of delamination was highest on PDMS-OH, next highest on PDMS-F, and virtually unchanged on PDMSCH3 (Fig. S9). This trend in increasing delamination as a function of increasing ionic strength followed the trend in surface charge, and further supported the idea that the EDL was playing an important role in the adhesion of crystals in our system. The same trend was observed for the rod morphologies, but the effect was less pronounced. We believe this difference could be a product of the slightly different surface chemistry of the facet the rods tended to rest on (the (11̅00) facet for rods versus the (0001) for discs and plates) and the different manners which the various morphologies responded to stress. For the case of plates on PDMS-OH cycled in water, we believe that the large increase in delamination (compared to rods or discs) can be explained by again considering that different morphologies responded to stress differently, and not to greater repulsion from the electrical double layer. Specifically, the plates, more so than discs or rods, partially delaminated (as permitted by their flexibility and high surface area, Fig. 5G-I and Video S3), allowing water to wet the interface and “propagate” delamination by replacing the less favorable crystal/PDMS-OH interface of the plate with the more favorable water/PDMS-OH interface. We further investigated the potential role of solvation effects by the addition of a surfactant (Tween 20). We expected that the introduction of a surfactant would lead to increased delamination of all crystals on all substrates because the amphiphilic nature of such additives can destabilize the crystal/PDMS interfaces through interactions with the crystals and/or the surface. Our observations, in terms of delamination in a surfactant solution versus water alone, were consistent with this hypothesis—all crystal morphologies on all surfaces delaminated at least 100% more so than in water alone (Fig. 4C). Crystals on PDMSOH experienced the most drastic increase in crystal delamination with at least 1400% greater delamination then that observed in water. We attribute this observation to the structure of the Tween 20, a polysorbate which includes four chains with repeating polyethylene glycol (PEG) units and only one long chained alkane. These PEG chains can interact favorably with both the crystal and PDMS-OH, destabilizing the crystal/PDMS-OH interface and thus increasing the number of delamination events. Additionally, the long-chained alkane moiety in Tween-20 can interact with non-polar surfaces, such as

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PDMS-CH3 and PDMS-F, thus destabilizing the crystal/PDMS-CH3 or crystal/PDMS-F interface. Crystal Assembly and Isolation via Selective Delamination. We leveraged our ability to tune adhesion using solution chemistry, surface chemistry, and stresscycling to yield 2D patterns of crystals through a process of “selective” delamination. To demonstrate this capability we synthesized a surface-chemical pattern with PDMS-OH and PDMS-CH3 regions,18 which were observed to have very different adhesion characteristics in Tween-20 solutions (Fig. 4C). Random deposits of crystals on this patterned surface would thus delaminate at different rates from the chemically distinct regions (higher rates of delamination on the PDMS-OH, Fig. 4C and Fig. 6A-D). Our results confirmed this expectation and we were able to drive the selective delamination of crystals from the PDMS-OH, generating patterns of crystals that matched the underlying chemistry (Fig. 6E-J and Fig. S10). We observed that, over 3,000 stress-cycles, discs resting on the PDMS-OH region delaminated over 92% of the time, whereas crystals resting on the PDMS-CH3 region delaminated only 7% of the time (Fig. 6K). This large difference in delamination ultimately led to the emergence of a clear pattern (Fig. 6G, J). This “selective” delamination is a simple form of assembly which takes advantage of surface-chemical differences and adhesion, circumventing techniques such as photolithography or microprinting (which require multiple steps and specialized equipment). An interesting aspect of this assembly method is the control over density offered—on average one crystal delaminates every four stress cycles from the PDMS-OH region, therefore terminating stress-cycling after discrete numbers of cycles offers the potential to control the number of crystals in each zone precisely.

Figure 6. Assembly of 2D patterns through selective delamination. A-D) Schematic illustration of the selective delamination process. A) Illustration of the chemical pattern where white indicates PDMS-CH3 regions and black indicates PDMS-OH regions. B-D) Crystals were randomly deposited. Stress-cycling of the substrate led to selective delamination and the emergence of a pattern. E, H) Optical micrographs of the chemically patterned PDMS with randomly deposited ZnO discs (no stress cycles have been applied). F, I) Optical micrographs after 1,500 stress cycles (ε = 1.0). There were ZnO discs located on both the PDMS-CH3 (the hexagon) and the PDMS-OH (interstitial area). G, J) Optical micrographs after 3,000 stress cycles (ε = 1.0). Virtually no ZnO discs remained on the PDMS-OH surface. K) Table comparing the area lost on PDMS-CH3 and PDMS-OH surfaces. All scale bars are 100 µm.

Another possible application for selective delamination would be to use differences in adhesion characteristics in order to bias a surface to “eject” one type of crystal while another type remained adhered. This type of surface could be used to isolate specific crystals from mixtures of different crystals. For example, we generated a heterogeneous substrate with a PDMS-OH surface neighbored by a PDMS-CH3 surface and then deposited a mixture of ZHS plates and ZnO discs (Fig. 7A, B, D), which, in Tween 20 solutions, had very different delamination characteristics on these respective surfaces (Fig. 4C). Specifically, ZHS

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adhered very well to PDMS-CH3, but not to the PDMSOH, whereas ZnO discs adhered very well to both surfaces. When we stress-cycled this chemically defined substrate (600 times at ε = 1.0), we observed that the PDMSOH was virtually cleared of ZHS plates while ZnO discs remained (Fig. 7C, E). Conversely, on the PDMS-CH3 roughly the same number of each morphology remained adhered. We analyzed this change by measuring the ratio of discs to plates: before stress-cycling this ratio was 7.4:1 and 5.9:1 on the PDMS-CH3 and PDMS-OH respectively; following stress-cycling this ratio was 7.7:1 and 62:1 for the PDMS-CH3 and PDMS-OH respectively (Fig. 7F), indicating a tenfold decrease in the number of plates relative to the discs on PDMS-OH after stress-cycling the substrate in a surfactant.

Figure 7. Isolation of ZnO discs from a mixture. A) Schematic illustration of the process. B, D) Optical micrograph of the crystals (ZHS plates and ZnO discs) near the border (indicated by the white dashed line) between the PDMS-CH3 (left side) and PDMS-OH (right side) before the support substrate was stress-cycled. C, E) Optical micrograph after the support substrate was cycled 600 times (ε = 1.0). F) Table comparing the number of crystals lost on the PDMS-CH3 to the PDMSOH. All scale bars are 50 µm.

We applied a similar strategy to the separation of other binary and even tertiary mixtures of crystals (Fig. S11 and S12). By expanding the range of surface and solution chemistries used, we expect that this strategy will make possible ever more sophisticated separations and/or patterning schemes that, for example, involve different morphologies and/or materials of different compositions. In a simple expansion, we improved the efficiency of the separation of tertiary mixtures using a “two-step separation” (Fig. S13). This two-step separation (with analogy to stepgradient separations) was designed by observing trends in the statistical similarities/differences of adhesion for dif-

ferent crystals on different surfaces in different environments using a color-coded, comparison table (Fig. S7)— this table enabled the identification of new separation strategies that were less apparent from the raw data. Specifically, we adopted a procedure which used water followed by a Tween-20 solution. Following this approach, we were able to improve the separation of discs from plates and rods by ~25% without expanding to different surface/solution chemistries (Fig. S13). 4. Conclusions In this work, we microcontact-printed various morphologies of ZnO and ZHS crystals onto the surfaces of chemically functionalized silicones, and monitored their displacement and delamination as the surfaces were stress-cycled. We showed that by controlling the chemistry of the substrate and the ambient environment, we could modify the displacement magnitudes and delamination characteristics of the crystals on the surfaces. We demonstrated that the crystal morphology and the interface area and chemistry were important to how the crystals migrated or detached from the elastic surfaces. We note that, save some important exceptions (e.g., plates on PDMS-OH in dry versus wet conditions), the statistical differences/similarities between the dry and wet environments were roughly symmetric, suggesting that it was the nature of the crystal/PDMS interface that dominated adhesion trends. An advantage of the approach taken in this study, which does not provide direct observations of the local, microscopic force balances at the crystal/polymer interface at the single particle level, was that it provided a convenient method to study a large amount of crystals and to enable predictive understandings of their ensemble behavior in various conditions. Further, we combined these understandings to assemble micronscale particles into discrete 2D patterns, and to separate mixtures of crystals through a process of selective delamination. We focused on PDMS, which was limited to magnitudes of strain on the order of ε = 1.8; however, other elastomers (e.g., Ecoflex) can support larger magnitudes of strain increasing the range of interfacial stress conditions that can be explored and potentially increasing the application of selective delamination to a greater range of crystal morphologies. Further, we expect that the concepts and methods described here could be applicable to other crystal systems that have different morphologies and/or chemistries. The strategies and understandings we advance in this work can be useful to fundamental investigations of hard/soft interfaces in the context of biomineralization or composite design, the design and synthesis of surfaces for stretchable electronics and sensors, and the development of new nano-/micro-crystal assembly techniques. Of particular interest is the ability to engineer surfaces with specific, mechanically-induced adhesion/anti-fouling characteristics. Such surfaces could be useful, for example, in

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forensics where the simple, non-invasive separation of mixtures of morphologically distinct particulate samples (e.g., soil or dust samples) is important,46 or in nanoscience where isolating and/or enriching materials of a specific morphology or chemistry could facilitate application and/or characterization.

ASSOCIATED CONTENT Supporting Information. Experimental details, additional experimental data, and supporting videos are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Department of Chemistry and the Nebraska Center for Materials and Nano Science (NCMN) at the University of Nebraska–Lincoln and the University of Nebraska– Lincoln for start-up funds. We thank the Nebraska University Research Council for support through a Faculty Seed Grant funded by the Layman Fund. S.A.M. thanks the NSF for support through the Nebraska EPSCoR FIRST Award program (EPS-1004094) and 3M for support through a NonTenured Faculty Award.

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Table of Contents Graphic


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Adhesion of Morphologically Distinct Crystals to and Selective

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