Multifunctional Microcavity Surfaces for Robust Capture and Direct Rapid Sampling of Concentrated Analytes

Evaporation patterns of liquid droplets containing nanoparticles or colloids have extensive applications in diagnostics and printing. Controlling these patterns by studying the evaporation behavior of colloidal droplets on surfaces is important for enhancing sensing platforms. In this study, A liquid‐repellent microcavity surface is introduced to robustly capture deposited analytic particles. The proposed microcavity surface maintains stable air pockets for liquid repellency and strong pinning for the spatial stabilization of the evaporating droplet, thereby resulting in a coffee‐ring concentration. This microcavity surface also acts as a “microcontainer” for the deposited particles, thereby protecting them against external damage. To demonstrate the multifaceted capabilities of microcavity surfaces, further comparison is done of three different surface structures, planar, micropillared, and that with microcavities in a hexagonal arrangement, by analyzing their evaporation dynamics and dried deposit patterns. The microcavity surface exhibits superior particle capture, thereby revealing its applicability in on‐site testing. Using the direct rapid sampling of analytical materials, the potential of the fabricated microcavity surface for point‐of‐care testing is demonstrated. The proposed microcavity surfaces suggest new avenues for the development of more robust and sensitive sensing platforms.


Introduction
[7][8][9] These patterns encompass various formations, including a simple ring (referred to as a coffee ring), multiple rings, homogeneous aggregations, and even cracked patterns. [1]Hence, employing strategies to regulate wetting and evaporation dynamics on engineered substrates can offer several advantages, such as a remarkable evaporation enrichment effect and new insights into analytical sensing platforms. [2,10]arious approaches have been proposed to exert control over the deposited patterns, which are influenced by a multitude of parameters including the evaporation mechanism, surface wettability and structures, flow dynamics, chemical additives, concentration, and particle geometry. [11,12]Among them, harnessing surface wettability to control dried deposits offered an effective and straightforward strategy for concentrating target particles (analytes) within droplets, thereby resulting in the development of sensitive sensing platforms.Numerous studies to control the wettability of droplets have been conducted. [13][16][17] In such research, structures found in nature are often mimicked to realize superhydrophobic surfaces. [18,19][22] By controlling the evaporation characteristics of droplets, it is possible to utilize them as a sensing platform.For instance, on superhydrophobic or slippery surfaces that exhibit a weak or eliminated pinning effect of the contact line, the particles within the droplets concentrated into small homogeneous spots as liquid evaporates, thereby enabling analyte aggregation to enhance the sensitivity for various sensing applications. [2,10]Conversely, an adhesive liquid-repellent surface exhibits a remarkable droplet-anchoring ability, known as the petal effect, which enhances particle concentration in the pinned area through evaporation-induced coffee-ring formation. [23,24]27] However, beyond the primary role of concentrating analytes, the multifaceted capabilities of a sensing substrate become increasingly crucial when considering its applicability in the real-world scenario. [28,29]In evaporation-induced particle aggregation or concentration, achieving spatial stabilization of the evaporating droplet is crucial for depositing target particles at precise locations for further analysis. [30,31]In this context, a micropatterned surface with both liquid repellency and strong adhesion is highly desirable to effectively anchor a drying droplet at the intended point of contact, while preventing undesired rolling, sliding, or movement on superhydrophobic or slippery surfaces.Furthermore, engineering surface microstructures with topological or chemical patterns can tailor the evaporation mechanism by altering the wettability and motion of the contact line during the drying process, thereby effectively manipulating the spatial distribution of the deposited particle patterns. [1,32,33]Our previous work demonstrated that such microstructured surfaces facilitated robust capture of aggregated particles from external mechanical contact, through the control of contact line dynamics during evaporation of sprayed microdroplets. [32]Consequently, the strategic engineering of microstructures on surfaces emerges as a versatile approach, offering a promising path for integrating multifunctionality into sensing substrates suitable for real-world applications.
In this study, to effectively capture the deposited particle patterns through evaporation-induced concentration, a novel approach is presented, which employs a stable liquid-repellent microcavity surface.The interconnected microcavities, characterized by their low surface area and enhanced mechanical robustness, play a pivotal role in maintaining air pockets [34] during droplet evaporation, thereby resulting in stable liquid repellency. [35]Additionally, microcavities exhibit strong adhesive properties along with strong contact line pinning.This combination serves as a physical means to immobilize the evaporating sessile droplets, promoting particle concentration or aggregation through droplet evaporation. [36]Further, the geometrical advantage offered by the interconnected microcavity surfaces acts akin to a "microcontainer," providing a protective structure that shields internally deposited analyte particles from external mechanical contact.
To underscore the efficacy of our proposed microcavity surface in achieving robust particle capture on sensing substrates, three types of surface structures are examined: smooth, micropillared, and microcavities featuring a hexagonal arrangement.These surface structures were chosen as model surfaces to probe the impact of wettability and topography on the evaporation dynamics and the resulting deposited patterns.We successfully illustrated the robust ability of the microcavity surface to capture the deposited analytical particles, particularly in protecting them against mechanical contact.By leveraging the efficient capture capabilities and mechanical robustness of the microcavity surfaces, analytical droplets and particles were directly sampled, thereby highlighting their potential for point-of-care testing (POCT).

Wetting Properties and Evaporation Dynamics
In this study, a microcavity design is adopted to concurrently achieve two key objectives: 1) maintaining long-term entrapment of air for stable liquid repellency and 2) ensuring robust capture of particles after droplet evaporation, as illustrated in Figure 1A.While the flat and micropillar-patterned surfaces, permit easy interconnection of air layers, the microcavities effectively encase the air pockets within isolated and separated cavities.39] Ultraviolet nanoimprint lithography is employed [40] (as shown in Figure 1B and described in the method section) to fabricate three distinct surface patterns: flat, micropillar-patterned, and microcavity surfaces.Additionally, to discern the effect of wettability, samples were fabricated using two materials: hydrophilic polyurethane acrylate (PUA) and hydrophobic polydimethylsiloxane (PDMS).][43][44] Figure 1C portrays the fabricated microstructured surfaces, featuring micropillars with a diameter (d) and pitch (p) of 4.5 and 40 μm, respectively, as well as the microcavities with a wall thickness (t) and hexagonal side length (l) of 3 and 40 μm, respectively.To investigate the wetting and evaporation dynamics of these samples in the context of sensing applications, a test system was designed using liquid droplets blended with nanoparticles, simulating the analyte-laden colloidal suspension.Specifically, we used polystyrene (PS) nanoparticles with a mean diameter of 200 nm, which were mixed in solutions with three different surface tensions.To adjust the surface tension while minimizing alterations in liquid viscosity, [45] solutions were prepared by mixing deionized (DI) water and isopropyl alcohol (IPA) (Figure S1 and S2, Supporting Information).Subsequently, the PS nanoparticles were diluted to a concentration of %10 10 particles mL À1 , resulting in three distinct liquid samples with surface tensions (γ) of 37.54 AE 1.16, 45.47 AE 2.61, and 67.97 AE 1.77 mN m À1 .
Figure 2 presents the data illustrating the wetting behavior of sessile droplets on each surface.The advancing contact angle (CA, θ a ), and receding CA (θ r ) on each sample, shown in Figure 2A, reveal that the microstructured surface, comprising micropillars and microcavities, consistently exhibits a higher CA value than that of planar surfaces, regardless of whether they are composed of hydrophilic and hydrophobic materials.This increased CA, showcasing the enhanced liquid repellency, is attributed to the small solid fraction present in the microstructured surface. [41,46]In Figure 2A, we show the comparison of the contact angle hysteresis (CAH), which represents the difference between the advancing and receding angles, among samples using three distinct liquid samples.[49] In other words, a larger CAH indicates stronger adhesion of a droplet to a surface, leading to resistance in the contact line. [48,50,51]y analyzing the measured CAH along with CA values, we confirmed that the microcavity surfaces exhibited both the highest CA and CAH, thus affirming their stable liquid repellency and strong adhesion to liquid droplets.
As shown in Figure 2B (for data on all surfaces, refer to the Figure S3, Supporting Information), the microcavity surface demonstrates the long-term entrapment of air within each isolated structure, which plays a crucial role in inhibiting the wetting transition from the Cassie to the Wenzel state.Consequently, stable liquid repellency was achieved in the proposed microcavity surface, regardless of the wettability of the constituent material and variation in the surface tension of the droplets.In contrast, the micropillar surfaces eventually underwent a wetting transition as the liquid penetrated the constituent microtextures, driven by thermodynamic equilibrium. [38]he subsequent results related to the evaporation dynamics and deposited particle patterns provide a detailed explanation of the highly adhesive microcavity surface with strong contact-line pinning.
To further investigate the dynamic wetting and evaportaion properties, we also characterized the time-dependent evolution of the CA and droplet diameter, as shown in Figure 2C-V.In this analysis, time t was normalized to the characteristic time t*, which corresponds to the moment when the CA reaches 0°.For hydrophilic surfaces, we observed that the sessile droplets evaporated with a constant contact radius (CCR) on both the smooth (Figure 2D,G) and microcavity surfaces (Figure 2F,I), due to strong contact line pinning.However, on the micropillarpatterned surfaces, the droplets initially evaporated in the CCR mode and then transitioned to the constant contact angle (CCA) mode (Figure 2E,H).In the case of hydrophobic planar (Figure 2N,Q) and micropillar-patterned surfaces (Figure 2O, R), reduced contact line pinning led to a nearly CCA mode during droplet evaporation, while the microcavity surfaces maintained the CCR mode owing to strong contact line pinning (Figure 2P,S).Furthermore, when the surface tension of the droplets decreased, f p decreased proportionally, thereby accelerating the transition from CCR to CCA, as shown in Figure 2E,H.These dynamic evaporation modes, including CCA, CCR, and CCR-CCA transition, are intricately influenced by the balance between the pinning force ( f p ) and depinning force ( f d ).The latter can be expressed as γ(cosθ À cosθ*), [49,52] where θ represents the dynamic CA and θ* denotes the apparent CA on each surface.During droplet evaporation on surfaces, the droplet boundary is initially pinned by the pinning force.This pinning mode can transition to a depinning mode with a decreasing contact radius, due to the driving force by the unbalanced surface tension arising from changes in CA.We defined the driving ratio (α) as the ratio of the depinning to the pinning force, f d /f p = (cosθcosθ*)/ (cosθ rcosθ a ).Thus, using the measured dynamic CA, we plotted the time evolution of α in Figure 2J-L,T-V.
For hydrophilic surfaces, α is less than 1 at the beginning of evaporation, indicating that the droplet tends to pin at the contact line, consistent with the sticky CCR mode.As evaporation progresses, f d increases due to a decrease in CA and results in increased α.On the hydrophilic planar and microcavity surfaces, α remained below 1 for most of the evaporation process, thereby ensuring a stable CCR mode.Meanwhile, when α exceeds 1, the droplet tends to transition to the slippery CCA mode.As shown in Figure 2A, the hydrophilic micropillar-patterned surfaces exhibited a smaller CAH value compared to that of microcavity surfaces.As a result, due to the smaller f p , α quickly increased relatively, leading to a transition in the evaporation mode from CCR to CCA.Furthermore, the wettability of the surface material significantly influenced the evaporation behavior of the sessile droplets.On the hydrophobic planar surfaces, the small CAH results in a high α (greater than 1), leading to the CCA mode for most of the evaporation process.In contrast, for the hydrophobic microcavity surfaces, α maintains a lower value, similar to hydrophilic materials, due to large CAH during evaporation process.As a result, they exhibit a stable CCR mode.Consequently, the driving ratio (α) plays a crucial role in influencing the contact line behavior of sessile droplets during the evaporation process, resulting in various observed evaporation modes, including the CCR, CCA, and CCR-CCA transitions.
More interestingly, for the hydrophobic micropillar-patterned surfaces, the existence of the Cassie-Wenzel wetting transition due to the geometric effect of protrusions may be critical in determining the evaporation mode.Figure 3 shows that the contact line behavior of the evaporating droplets is significantly affected by both the surface tension of the liquid and the presence of microstructures on the substrates.The microscale geometry and surface topography of the microcavity surface provide favorable conditions for pinning the contact line of the droplet, resulting in a larger CAH compared with that of the micropillarpatterned surface.A microcavity surface features concave structures with sharp edges, which effectively trap the liquid within isolated cavities, generating a stronger pinning effect. [53]This enhanced CAH leads to a stable CCR mode, as confirmed in the dried feature shown in Figure 3A.In contrast, the presence of micropillars on the surface creates an array of small convex protrusions with round edges, which may decrease the energy barrier required for depinning by increasing the solid-fluid wetting contact area (Figure 3B). [54]This phenomenon occurs not only on hydrophobic surfaces, but also identically on hydrophilic surfaces (Figure S4, Supporting Information).This reduction in resistance facilitates the movement of the evaporating liquid, resulting in the CCA mode despite a small α value below 1.The experimental evidence shown in Figure 3 underscores that the dynamics of the contact line depend on the surface topography and geometry of the microstructured surfaces, ultimately affecting the evaporation behavior.For a more detailed visual representation, refer to the videos in Video S1-S6 and Table S1-S5, Supporting Information.

Deposited Particle Patterns
Figure 4i shows how the deposited particle patterns are affected by the evaporation behavior, depending on the wettability, topography, and surface tension of the surfaces.On the hydrophilic planar surface (Figure 4A(i)-C(i)), we observed the clear coffeering formation of deposited particles with a larger area.This occurred owing to the CCR mode with a small CA during the evaporation process of the colloidal droplet.On the hydrophilic micropillar-patterned surface, as shown in Figure 4D(i)-F(i), the evaporation mode transitions from CCR to CCA, thereby resulting in the formation of multiple ring patterns [1] On the hydrophilic microcavity surface (Figure 4G(i)-I(i)), the CCR mode with a large CA created a regular-shaped and homogeneous deposition of nanoparticles in a smaller area.On the hydrophobic planar surface, the colloidal droplet exhibited concentration with either the CCR-CCA transition or nearly the CCA mode depending on the surface tension of the liquid, thus resulting in the formation of a homogeneous aggregate with irregular shapes in a confined region (Figure 4J(i)-L(i)).Finally, on the hydrophobic micropillar-patterned surface, as shown in Figure 4M(i)-O(i), similar homogeneous and concentrated deposit patterns were also found owing to the evaporation mode of CCA, with contact line depinning.
However, on the hydrophobic microcavity surface, the particles aggregated only at the contact line of the evaporating droplets owing to the CCR mode and stable air entrapment during the evaporation process (Figure 4P(i)-R(i)).[57][58] On the hydrophilic microcavity surface, the particles were uniformly deposited on all the wetted areas of the microcavities in the CCR mode owing to the formation of a concave-shaped contact line caused by the hydrophilic nature of the constituent material (the typical Wenzel state). [32]Contrarily, although the apparent evaporation mode of the sessile droplet on the hydrophobic microcavity surfaces was clearly the CCR mode owing to the strong contact line pinning caused by the concave features and sharp edges of the microcavity topography, the liquid was unable to penetrate the concave-featured microcavity at the microscopic level.This is because air tends to be stably trapped within isolated microcavities owing to the hydrophobic nature of the constituent material.This unique behavior enables most particles to be deposited and aggregated at the contact line of the evaporating sessile droplets, thereby resulting in the formation of a distinct particle concentration inside the microcavities owing to the coffee-ring effect in the small contact area of the evaporating droplets, as depicted in the schematics of Figure 5.The stable entrapment of air on the hydrophobic microcavity surface became more evident than the deposited patterns observed on the hydrophobic micropillarpatterned surfaces, as shown in Figure 4M(i)-O(i).Here, the protrusion of the micropillars can induce a catastrophic wetting transition from the Cassie to the Wenzel state, particularly in the presence of localized defects or damage. [59]This transition causes the liquid to rapidly penetrate the surface texture, thereby resulting in uniform deposition along the final contact line after evaporation.
While the aforementioned discussion provides insights into the evaporation dynamics and resultant coffee-ring concentration of the deposited particle patterns on microcavity surfaces, the protection of the concentrated and separated particles from unintended external damage must also be analyzed; this would ensure real-life applications in actual environments.To examine whether the microcavity surfaces prevented analytical particle loss caused by unintended mechanical contact, we conducted a qualitative evaluation of the robustness of the captured particles on the surfaces using the tape-peeling test. [32]All the test surfaces were gently touched and peeled off using adhesive tape three times, and optical (Figure 4(ii)) and fluorescence (Figure 4 (iii)) images were captured to qualitatively assess the amount of the particle residue on each surface after mechanical contact.These images presented in Figure 4 (ii-iii) show that the planar and micropillar-patterned surfaces lost particles, whereas the microcavity surfaces effectively preserved most of the deposited particles for the hydrophilic and hydrophobic materials.Although the particles on the microcavity surfaces were either uniformly deposited on the droplet contact area or localized at the contact line of the droplets, most particles were captured inside the isolated cavity structures.During the mechanical contact transfer of particles in the tape-peeling test, the soft materials (in this case, the thin tape was simulated as a soft material) were deformed under an external force, thereby causing the particles to be pulled out of the microstructures.Here, the concave and interconnected features of the microcavity surfaces serve as a protective structure that robustly resists the deformation of the contact materials, thereby improving the capture ability of the internally deposited and aggregated particles.However, the protrusion and isolated features on the micropillar-patterned surfaces readily allow the deformation of the contact materials, thereby enabling the removal of particles captured by the microstructures.This implies that the proposed microcavity surface can act as a "microcontainer" to preserve analytes or particles.
In addition to microfluidic or sessile droplet-based analytical platforms, direct sampling, which was performed without requiring micropipettes or intricate fluidic devices, can considerably reduce the sampling time and is the preferred approach for POCT applications.As such, micropipette-free direct sampling for on-site detection requires mechanical durability to withstand a certain degree of contact pressure when collecting analytical solutions or materials, as well as the ability to rapidly and stably capture analytical particles.To verify direct rapid sampling in POCT applications, we illustrated the capability of microcavity surfaces to capture analyte solutions and particles by harnessing the microcontainer concept. [60,61]To simulate direct sampling, we attached the surfaces to a nitrile glove, as portrayed in Figure 6A.Droplet sampling was further performed by the direct contact of the surfaces with a pristine analytical solution.
Although the overall dried patterns on the surfaces exhibited a similar behavior after the evaporation of the sessile droplets (Figure 4), the micropillar-patterned surfaces suffered severe mechanical damage owing to the contact pressure during the direct droplet sampling process (Figure 6B).Such damage may result in reduced reliability and a higher risk of contamination of the analytical particles in the direct sampling approach.Conversely, Figure 6B shows that the microcavity surfaces consistently retained their structural frames under an external contact pressure, thereby effectively trapping the particles deposited within the cavity structures.This suggests that the  interconnected microcavity surface acts as a mechanical armor, thereby preventing the removal of fragile analyte particles and providing a robust structural frame that withstands external damage. [62]Consequently, we expect that microcavity surfaces, alongside their functional effectiveness and robust structural design, would be well suited for use in the real world, particularly in wearable and portable sensing devices operating in high-contact pressure environments.

Conclusion
In this study, we systematically investigated the evaporation behavior of sessile droplets on planar, micropillar, and microcavity surfaces using different liquid surface tensions, wettabilities, and geometries.Our findings revealed that the surface topography and wettability considerably affected the evaporation process of the droplets, thereby resulting in different particle deposition patterns after drying.This insight is essential to understand the influence of the surface properties on droplet evaporation and particle deposition, which is critical for different applications.Particularly, we effectively illustrated the capability of stable liquid-repellent microcavity surfaces with strong contact line depinning to firmly anchor the sessile droplets containing the analytical particles.These microcavity surfaces facilitated the localization of particles at the contact line of the droplets through the coffee-ring concentration phenomenon, thereby resulting in the robust capture of concentrated particles in their cavity structures preventing mechanical damage.The microcavity surfaces exhibit promising potential as microcontainers, effectively containing analytical particles deposited on the surfaces.Furthermore, by harnessing the interconnected network and stable air entrapment of the hydrophobic microcavity surface, we effectively demonstrated the ability of the microcavity surfaces to capture analyte droplets for rapid on-site sampling, thereby rendering them particularly suitable for POCT in the near future.We expect this innovative approach to have practical implications on the on-site analyte concentration, thereby making it a promising tool for low-cost disease diagnostics in resource-limited environments (e.g., as a sensing substrate for surface-enhanced Raman spectroscopy platforms).
While the approach presented in this study is simple and effective for the robust capture of concentrated analytical particles, there are certain limitations that need to be further explored and addressed in future research.The evaporation behavior of the droplets is influenced by different environmental conditions, including humidity, temperature, and airflow.Additionally, the composition of the evaporating droplets, comprising complex components such as biological, organic, or inorganic materials, can considerably affect the evaporation process and the resulting patterns of the deposited particles.Hence, we believe that gaining a deeper understanding of the interplay between these factors, including the microstructure, droplet composition, and environmental conditions, is crucial for effectively controlling the evaporation processes of the particle-laden droplets and resulting drying patterns for the advancement of droplet-evaporation-induced analytic platforms.Further, when analytical particles are deposited on a surface with the movement of the contact line or area, the presence of the accompanying contaminants becomes inevitable. [2,63]These contaminants can influence the droplet evaporation mode and deposition patterns, thereby resulting in a reduced repeatability of and reliability on the analytical substrate.We anticipate that the coffee-ring concentrated patterns achieved through contact line pinning in the CCR mode would be preferable than the small particle aggregates formed by eliminating contact line pinning in the CCA mode.Based on the findings of our study, the moving distance of the contact line during transition from the CCR-CCA transition or in the CCA mode is considerably longer than that in the CCR mode.This observation suggests that stable contact line pinning in the CCR mode is advantageous in reducing the potential of increased concentrations of contaminants during evaporation.Hence, the issue of contaminants during evaporation has to be carefully considered and addressed to ensure accurate and consistent results on the analytical platforms.Hence, to ensure accurate and consistent results on the analytical platforms, the issue of contaminants during evaporation must be carefully considered and addressed.We believe that the proposed engineered surfaces not only offer insights into the physical mechanisms of droplet evaporation and deposited patterns but also hold the potential to serve as simple and commercialized substrates for analytical sensing platforms.

Experimental Section
Silicon Mold Fabrication: Hexagonal wall structures with a width, height, and wall length of 3, 9, and 40 μm, respectively, were fabricated.The pillar structures were fabricated with a diameter and height of 4.5 and 8.8 μm, respectively, in a hexagonal arrangement with a pitch of 40 μm.Si molds were fabricated using photolithography and reactive ion etching (RIE).
Imprinting Process: Microstructures were fabricated using a polyurethane-acrylate (PUA) resin (MINS-311RM resin, Minuta Technology) as the hydrophilic surface and polydimethylsiloxane resin (MINS-UV-PDMS, Minuta Technology) as the hydrophobic surface.The PUA or UV-PDMS resin was dropped onto a polyethylene terephthalate (PET) film (125 μm), and the Si mold (OTS coated, 120 °C, 30 min) was pressed slightly against the liquid drops.The PUA and UV-PDMS resins were cured under UV light (λ = 365 nm) for 2 and 70 s, respectively.
Contact Angle Hysteresis: The surface CAH was measured using a contact angle meter (SmartDrop, Femtobiomed Inc., Korea).The CAH was measured using a captive method.The advancing CA was measured by ejecting the droplet while the droplet was in contact with the needle (15-20 μL); the receding CA by sucking the droplet in a similar state.A 20 μL droplet was brought into contact with the surface, and the receding CA was measured while sucking the liquid.
Evaporation and Particle Aggregation Tests of Sessile Droplets: The CAs and diameters of the surfaces were measured using a contact angle meter (SmartDrop, Femtobiomed Inc., Korea).A suspension droplet of %5 μL was deposited on the planar PUA and PDMS surfaces using the automatic micropipette of the liquid-droplet analysis tool.Once the droplet was deposited on the surface, droplet evaporation was recorded using a contact angle meter equipped with a high-resolution camera.The environmental temperature and relative humidity (RH) were maintained at 22.24 AE 0.68 °C and 47.32% AE 3.12%, respectively.Each experiment was repeated at least five times to ensure reproducibility.To suppress the influence of convection during droplet evaporation, we fabricated a cell and fitted it onto the equipment (Figure S5, Supporting Information).
Particle Aggregates Analysis: Images of the particle aggregates on the hydrophilic and hydrophobic structures were obtained using an optical microscope (Keyence, VK-X1000).
Tape-Peeling Test and Fluorescent Imaging for Robust Capture Ability: A tape-peeling test was performed to calculate the robust capture ability.Experiments were performed using scotch (3 M, USA) on the samples that underwent evaporation and particle aggregation tests.A scotch magic tape was used for the peeling test and pressed with a finger.This procedure was repeated thrice (The Scotch tape was applied with a force of 5-10 N and removed at a speed of 5-7 mm s À1 while maintaining a 90°angle with the surface, similar to the ASTM D3330 test method F).After separating the two surfaces, we obtained 3D profile images of the planar and 3D microstructured surfaces at similar points using a laser scanning confocal microscope (Keyence, VK-X1000) and fluorescent images of the planar and 3D microstructured surfaces using a fluorescent confocal microscope (K1-Fluo, Nanoscope Systems Inc., Korea).For comparison, fluorescence images were captured under similar conditions (laser power: 30%, objective lens 10X, Green gain value: 253, average of 20 measurements).The tape was peeled off to separate the surface while maintaining a direction perpendicular to the film.
Droplet-Capturing Surface for Rapid Sampling: The rapid sampling of the droplets was tested using a finger.PS microspheres (FluoSpheres Carboxylate-Modified Microspheres, 0.2 μm, yellow-green fluorescent (505/515), 2% solids-F8811) were dispersed in DI water.The nanoparticle was diluted to %10 10 mL À1 (equivalent to 0.005%) and dispersed at 700 RPM using a digital stirrer (PC-420D, Corning, USA).The surface of the glass substrate was cleaned with IPA and DI water.A 10 μL droplet was sprayed onto the glass substrate using a pipette.The planar microstructured film was fixed on the finger using tape and lightly pressed for 10 s.A 3D profile image of the droplet on the film was obtained using a laser scanning confocal microscope by slowly lifting the finger.

Figure 1 .
Figure 1.A) Schematic showing the strategy of the microcavity surfaces for enhancing liquid-repellency and particle capture by housing the air pocket and analytic particles within a protective interconnected microstructure; B) Imprinting process to fabricate microstructured surfaces; C) Microstructures with hexagonal arrangement of micropillars and microcavities.

Figure 2 .
Figure 2. Dynamic wetting behavior and evaporation dynamics by varying the microstructures, surface wettability, and surface tension of colloidal droplets.A) Contact angle hysteresis (CAH) of the liquids with different surface tensions on the smooth and microstructured surfaces.Here, the error bar stands for the standard deviation of five measurements; B) Captured time-dependent images of evaporating droplets in planar, micropillar-patterned, and microcavity surfaces.C,M) Schematics for evaporation mechanism of hydrophilic and hydrophobic surfaces: i) planar, ii) pillar, and iii) cavity.D-F,N-P) Time evolutions of contact angle, G-I,Q-S) contact radius, and J-L,T-V) driving ratio on planar, micropillar-patterned, and microcavity surface for hydrophilic and hydrophobic materials.The dashed lines indicate the experimental CCR-CCA transition.

Figure 3 .
Figure 3. Captured optical images during evaporation process.A) Hydrophobic microcavity structure; B) Hydrophobic micropillared structure.Here, all the scale bars are 400 μm.

Figure 4 .
Figure 4. i) Optical images of aggregated nanoparticles based on the microstructures and surface tensions of droplets, ii) optical images after tapepeeling test, and iii) fluorescence images after tape-peeling test.A-C) Planar, D-F) micropillar-patterned, and G-I) microcavity surfaces fabricated using hydrophilic material.J-L) Planar, M-O) micropillar-patterned, and P-R) microcavity surfaces fabricated using hydrophobic material.Here, all the scale bars are 500 μm.

Figure 5 .
Figure 5. Schematics for dried deposit patterns of A) hydrophilic and B) hydrophobic surfaces: i) planar, ii) pillar, and iii) cavity.

Figure 6 .
Figure 6.Droplet-capturing microstructured surfaces for direct rapid sampling: A) Dip-pull manipulation for the on-site sampling of liquid phased analytes.Here, all the scale bars are 10 mm.B) Deposited particle patterns on microstructured surfaces after the evaporation of captured droplets.Here, the scale bars are 500 μm except 50 μm bars.