Hydrophilic Confinement Research Articles

Self-Diffusion in Confined Water: A Comparison between the Dynamics of Supercooled Water in Hydrophobic Carbon Nanotubes and Hydrophilic Porous Silica.

Confined liquids are model systems for the study of the metastable supercooled state, especially for bulk water, in which the onset of crystallization below 230 K hinders the application of experimental techniques. Nevertheless, in addition to suppressing crystallization, confinement at the nanoscale drastically alters the properties of water. Evidently, the behavior of confined water depends critically on the nature of the confining environment and the interactions of confined water molecules with the confining matrix. A comparative study of the dynamics of water under hydrophobic and hydrophilic confinement could therefore help to clarify the underlying interactions. As we demonstrate in this work using a few representative results from the relevant literature, the accurate assessment of the translational mobility of water molecules, especially in the supercooled state, can unmistakably distinguish between the hydrophilic and hydrophobic nature of the confining environments. Among the numerous experimental methods currently available, we selected nuclear magnetic resonance (NMR) in a field gradient, which directly measures the macroscopic translational self-diffusion coefficient, and quasi-elastic neutron scattering (QENS), which can determine the microscopic translational dynamics of the water molecules. Dielectric relaxation, which probes the re-orientational degrees of freedom, are also discussed.

Two-dimensional sub-nanometer confinement channels enabled by functional carbon dots for ultra-permeable alcohol dehydration

Graphene oxide (GO) membranes have shown great promise in pervaporation dehydration of biofuels, expected to satisfy the demand of lower energy consumption. However, the swelling of interlayer spacing between GO nanosheets in aqueous environment, usually leads to deteriorated membrane separation performance and limited stability. Here, we report ultra-permeable sulfonated carbon quantum dots (SCDs)/GO composite membranes that are designed through strategically introducing the functional quasi-spherical nanoparticles - SCDs as water transport promoters and crosslinker into GO laminates. Well self-assembly stacked manner between SCDs and GO nanosheets created highly hydrophilic two-dimensional sub-nanometer confinement interlayer channels for fast water transport, while the crosslinking effect between SCDs and GO nanosheets producing a stable microstructure to maintain GO interlayer spacing against swelling. Furthermore, the SCDs occupied the vacancies around the ordered edges of GO nanosheets also resulted in an enhancement of water selective permeation. The microstructure evolution was confirmed by low-field nuclear magnetic resonance spectra. The prepared SCDs/GO membrane on polyethersulfone (PES) substrate with ∼150 nm selective layer exhibited a total flux of 5.88 kg m−2 h−1 and water/butanol separation factor of ∼4407 at 343 K, which outperforms the performance of state-of-the-art membranes reported to date. Meanwhile, the thin composite membrane showed long-term operation stability for 130 h when fed with 90 wt% butanol/water solution. This research provides a promising method for the selective water separation of high-performance GO membranes.

Controlled Water Uptake in Fuel Cell Membranes with Dual Chemistry Confinement

Nafion is one of the most commonly used materials for proton exchange membranes in polymer-electrolyte fuel cells (PEFC) due to its stability and high ion-transport capabilities. However, its dehydration problem as well as dimensional instability limits its application in high-temperature, low-humidity environments. In this paper, we propose a potential solution to this problem by introducing nanoscale confinements to the Nafion chains. In particular, a coarse grain molecular dynamics model is developed that reproduces the water absorption curve of the pristine Nafion membrane. The model enables us to perform systematic studies on the effects of confinement to the absorption behavior of the Nafion membrane at different activities. Confinement of different chemistries and sizes has been introduced in the simulation system in order to investigate its effects on water absorption on different activities. It is found that confinement of dual chemistry (consisting of both hydrophobic and hydrophilic confinements) offers a solution to its drawbacks. The hydrophobic confinement helps flatten the absorption curve and decrease the water absorption at high activity, while the hydrophilic confinement acts as a water reservoir, which increases the water absorption at low activity. The Nafion membrane with dual chemistry confinement in general has higher water absorption with smaller dimensional instability, which makes it suitable to operate in a high-temperature, low-humidity environment.

On the role of confinement plate wettability on pool boiling heat transfer

Remarkably high heat transfer coefficient observed during boiling makes it one of the most sought-after thermal management solutions. However, the heat dissipation potential deteriorates in practical scenarios of confined spaces typically encountered in electronic packages and compact heat exchangers, among others. Contrary to unconfined pool boiling, confinement hinders buoyancy aided vapor removal from the heater surface. Accordingly, vapor crowding inhibits the continuous supply of fresh liquid to the heater surface, eventually resulting in premature dryout/critical heat flux (CHF) in comparison to the baseline case of unconfined boiling on the same surface. While studies in literature report the effect of various parameters such as gap dimensions, heater surface to confinement plate area ratio, and surface inclination on confined pool boiling, the role of confinement plate wettability on bubble behavior and heat transfer has yet not been reported. Here we perform controlled experiments to show that superhydrophobic (SHPo) surfaces attract vapor/air bubbles and can be used to enhance heat transfer during confined boiling. In comparison to the typical case of boiling with hydrophilic (HPi) confinement plate, nucleating bubbles underneath a SHPo confinement plate detach easily from the heater surface to provide a continuous pathway for liquid re-supply and heater surface rewetting. Using this strategy, we shown a significant increase of up to 82% in heat dissipation capacity using SHPo confinement plate in comparison to the baseline case of the HPi confinement plate. Detailed parametric investigation of the effect of confinement gap and heater surface to confinement plate area ratio on heat transfer performance is also reported. We believe that the ability to improve confined boiling heat transfer performance by merely tuning the wettability of the confinement plate will find wide applications in two-phase heat sinks for high heat flux applications.

Influence of Water on Structure, Dynamics, and Electrostatics of Hydrophilic and Hydrophobic Ionic Liquids in Charged and Hydrophilic Confinement between Mica Surfaces.

Water is ubiquitous in the environment and is the origin for operational constraints in ionic-liquid based electrolytes for supercapacitors. In this study, the influence of water on the interfacial behavior of hydrophilic (1-ethyl-3-methylimidazolium ethylsulfate, abbr. [EMIM][EtSO4]) and hydrophobic (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, abbr. [EMIM][FAP] and [EMIM][TFSI], respectively) ionic liquids (ILs) confined between mica surfaces was investigated at separations precisely modulated by a surface force apparatus and at controlled relative humidity between 0% and 50% RH. Diffusion experiments revealed that water spontaneously invades the nanoconfined ILs above a certain humidity threshold and that the confined hydrophobic IL is completely replaced by water at sufficiently high environmental humidity (∼45% here) as a result of surface-induced phase separation. This behavior is expected to be universal for other ILs that are not fully miscible with water when they are confined in hydrophilic nanopores of a few nanometers in width. The effect of environmental humidity on interfacial structure, dynamics, and electrostatics was studied via dynamic force measurements. In the dry state, several layers of ions are immobilized on the mica surface, and the effective viscosity increases by up to 2 orders of magnitude with a decrease in film thickness from ∼10 to ∼3 nm. Based on recent work, it is proposed that nanoconfinement enhances the anion-cation association in highly concentrated electrolytes, thereby justifying the loss of fluidity of the ILs. When phase separation is excluded, water is intercalated in the layered structure of the three ILs, and it leads to a change of the layer thickness compared to the dry state. Furthermore, our results reveal that interfacial water prevents ions from being immobilized on the surface and facilitates the outflow of both hydrophilic and hydrophobic ILs by reducing their effective viscosity in the order [EMIM][FAP] < [EMIM][TFSI] < [EMIM][EtSO4]. The underlying mechanisms are evaluated by considering the roles of water in enhancing ion dissociation through screening of electrostatic interactions and solvation of the selected ILs to different extents. The discussed experimental observations support the recent discoveries made by molecular dynamic simulations and neutron scattering studies that using hydrophilic ILs coupled with water as cosolvent could lead to the enhanced power density of IL-based supercapacitors, and therefore, that water-in-(hydrophilic) ILs is a direction worth exploring as electrolytes for supercapacitors.

Microphase separation of a miscible binary liquid mixture under confinement at the nanoscale

Recent experimental works suggested that the confinement into a cylindrical nanopore induced the microphase separation of a binary liquid, despite the miscible character of its bulk counterpart. A core–shell organization was evidenced such that one of the liquids was strongly anchored to the solid surface whereas the other was confined at the center of the pore. At the same time, a study based on atomistic simulations suggested a strong heterogeneity and the absence of a separation. In this work, by refining the solid–liquid interactions to qualitatively reproduce the experimental adsorption isotherms of both single liquids, the microphase separation and the core–shell structure are captured. By tuning the surface chemistry of the nanopore to mimic hydrophilic and hydrophobic confinement, we show that it is possible to control the structural characteristics of the core–shell structure.The molecular origin of the microphase separation is then ascribed to the strong hydrogen bonds and a commensurate arrangement between the confining material and both liquids.

One-lens camera using a biologically based artificial compound eye with multiple focal lengths

This work presents the fabrication of a one-lens camera using a biologically inspired artificial compound eye with multiple focal lengths. Traditional camera designs that consist of many separated lenses are difficult to assemble due to tight tolerance. The one-lens camera design is demonstrated experimentally in this work to avoid tolerance buildups. This structure is based on the principles of both the human eye as well as an insect’s compound eye. The artificial compound eye is a curved hexagonal microlens array, like an ommatidial array, wherein each artificial ommatidium collects light with a small angular acceptance. The ommatidia, in a typical hexagonal arrangement of 37 lenses, are arranged across a hemispherical photopolymer dome. The curved hexagonal array helps us to achieve a compact and wide field-of-view camera module. The fabrication process for the curved array is divided into two parts: creation of a planar hexagonal multi-lens array, and a replication process. To create the planar array, we use inkjet printing technology with the hydrophilic confinement effect to establish microlens shapes with different profiles. Next, the replication process converts the planar array into a curved shape. The spherical configuration of the hexagonal array is accomplished by applying the template architecture to a reconfigurable surface shape, that is, a photopolymer duplication using a deformed elastomer membrane with the hexagonal array pattern. In our experimental demonstration, microlenses in four rings with different focal lengths are fabricated on a single hemispherical lens with radius of curvature of 2.4 mm. The thickness of our proposed system is 3.04 mm, the f-number is 1.68, and the diagonal field of view is 92.6 deg. Above all, our presented camera module system uses a single one-piece lens.

Liquid Water Confined in Cellulose with Variable Interfacial Hydrophilicity

Abstract We investigate liquid water confined within nanoscale cellulose slabs by means of molecular dynamics simulations. Depending on the construction of the cellulose–water interface, two different surface structures with distinct levels of hydrophilicity are exposed to the water. The different philicities are reflected in the response of the water phase to this geometric confinement, both in terms of the density profile and in the strength of the aqueous hydrogen bonding network. At the smooth surface cut along the (010) axis of the cellulose crystal, water shows typical properties of a hydrophilic confinement: the density shows fluctuations that disappear further away from the wall, the water molecules orient themselves and the coordination numbers increases at the interface. As a consequence, the water becomes “harder” at the interface, with a considerably increased local ordering. At the zigzag-shaped surface along the (111) axis, the degree of hydrophilicity is reduced, and only small effects can be seen: the density shows weak fluctuations, and the orientation of the water molecules is closer to that of bulk water than to the smooth surface. The local coordination numbers remains constant over the whole confinement. Our work shows that the nature of the exposed cellulose interface has a strong influence on how the structure of adjacent water is modified. The different ways of surface construction yield distinct degrees of hydrophilicity and spatial accessibility regarding the hydrogen bond network, resulting in a notably different interfacial water structure.

Deconstructing the Confinement Effect upon the Organization and Dynamics of Water in Hydrophobic Nanoporous Materials: Lessons Learned from Zeolites

The properties of confined water are relevant to many chemical, geological, and biological phenomena, where they underpin essential changes to molecular scale reactivity–perturbing both the energetic and configurational landscape. Though much prior literature has focused on hydrophilic confinement, the hydrophobic confinement of water is less well understood. Here, we use molecular dynamics simulations to investigate the structures and dynamics of water in hydrophobic all-silica zeolites that have sequentially smaller pore dimensions. Of special interest is the role that pure geometric restriction imparts, relative to the rugged potential energy landscape for water interacting with the atomistic pore surface. These two effects were studied via the hydrogen bond dynamics, specifically the rates and mechanisms of hydrogen bond breakage and formation. Measuring the dynamic features as a function of scaling the water:zeolite interaction energy revealed that geometric restriction is responsible for 67%–86% of ...

Thermodynamic Anomalies in Stretched Water.

Via molecular dynamics simulations of the TIP4P/2005 water model, we study liquid water's anomalous behavior at large negative pressure produced through isochoric cooling. We find that isochores without a pressure minimum can display "reentrant" behavior whereby a system that cavitates upon cooling can then rehomogenize upon further cooling. This behavior is a consequence of the underlying density maximum along the spinodal, but its actual manifestation in simulations is strongly influenced by finite size effects. These observations suggest that water under strong hydrophilic confinement may display richer phase behavior than hitherto assumed. This also suggests that propensity toward cavitation does not always correlate with greater tension, contrary to the prevailing assumption for interpreting water stretching experiments. We also show that a maximum spinodal density in water results in a locus of maximum compressibility and a minimum speed of sound that are independent from any influence of a liquid-liquid critical point (LLCP). However, we demonstrate that structural signatures of a Widom line, which likely emanates from an LLCP at elevated pressure, extend to large negative pressure, but such signatures are only observed upon sampling water's underlying potential energy landscape, rather than the thermalized metastable liquid.

Mediating gold nanoparticle growth in nanoreactors: Role of template-metal interactions and external energy

The unique properties of gold at the nanoscale have favored application in a variety of fields from catalysis to targeted drug delivery; moreover, the reactive nature of gold nanoparticles allows its use as an active center for nanoreactors. This study presents the controlled synthesis of unsupported nanoreactors with self-assembled polymers as template and gold incorporated as active centers. Gold (I) chloride was found to react with the hydrophilic domain of the template and produce metal gold sheets of 30–50 nm in size, as well as with the hydrophobic cavity to produce metal gold clusters, 2–3 nm in size. The size of gold nanostructures was characterized by High Resolution TEM, while the interaction between gold and polymer was confirmed by molecular modelling. The careful spectroscopic characterization including UV–Vis and IR allowed to differentiate clusters and sheets and to follow the growth of the gold particles. The control of the environment surrounding the active center (hydrophilic versus hydrophobic domains) is essential for the development of tailored nanoreactors. The control of the interaction between the polymer and gold was obtained by modifying the nature of the polymer and external energy. It was found that the presence of monomethyl ester in the polymer intensifies the interaction of gold on the hydrophilic domain while producing larger gold sheets (both hpc and fcc) and slightly smaller gold clusters. Sonication was found to disrupt the whole self-assembly process and completely eliminate the interaction of gold with the hydrophobic cavity producing exclusively atomically thin layers of hpc gold. The ability to control the interaction of gold with the hydrophobic or hydrophilic cavity offers tremendous potential for many applications from drug delivery to nanocatalysis in hydrophilic and hydrophobic confinement.

Variable focus microlens array with curved electrodes

In this paper, a liquid crystal microlens array with a curved electrode is designed and fabricated. The fabrication process consists of two parts: fabricating the microlens array and assembling the liquid crystal cell. The first process utilizes the hydrophilic confinement effect, an inkjet printer, and the replication process to fabricate a microlens array on a glass substrate. A transparent, organic, conductive poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) is spin-coated on the microlens array as a curved electrode, and the microlens array is further flattened by SU-8 photoresist. It is then assembled with indium tin oxide glass. Interference patterns for the liquid crystal microlens array are measured and agree well with theoretical calculations. From interference patterns, the focusing power range is measured to be from −47.28 to −331 diopters under 10 V, which corresponds to focal length from −2.12 cm to −0.3 cm at 532 nm wavelength. This technology could be useful for optical zoom systems or focus-tunable lens applications.

Structure and Dynamics of Ionic Liquid [MMIM][Br] Confined in Hydrophobic and Hydrophilic Porous Matrices: A Molecular Dynamics Simulation Study.

The effects of confinement on the structural and dynamical properties of the ionic liquid (IL) 1,3-dimethylimidazolium bromide ([MMIM][Br]) have been investigated by molecular dynamics simulations. We used zeolite faujasite (NaY) as a hydrophilic confinement and dealuminated faujasite (DAY) as a hydrophobic confinement. The presence of an extra framework cation, [Na+], in NaY makes the host hydrophilic, whereas DAY, with no extra framework cation, is hydrophobic. Although both NaY and DAY have almost similar structures, the IL showed markedly different structural and dynamical properties in these confinements and in bulk. In the confinements, the cation-cation radial distribution function, which strongly depends on temperature, exhibits a layer-like structure, whereas in bulk, it shows a liquid-like structure that hardly depends on temperature. Although the interaction between [MMIM]+ and Br- in DAY is stronger than that in both NaY and bulk, the strength of the interaction between them is almost invariant with temperature. Both [MMIM]+ and Br- strongly interact with Na+ of the host, and their interaction strongly depends on temperature, whereas the interaction of the IL with Si and O is very weak and invariant with temperature. In bulk, the self-diffusion coefficient, [D], of both [MMIM]+ and Br- increases exponentially with temperature, and the D of the cation is slightly higher than that of the anion at all studied temperatures, whereas in the confinements, [MMIM]+ moves much faster than Br-. For example, in the hydrophilic confinement, the D of the cation is 20-30 times higher than that of the anion. The D of both the ions decreases significantly in the confinements as compared to that in bulk. During diffusion, [MMIM]+ diffuses closer to the inner surface in the hydrophilic confinement than that in the hydrophobic confinement. The diffusion pathway imperceptibly depends on temperature but strongly depends on the nature of the confinement. The self part of the time-dependent van Hoove correlation function of [MMIM]+ in the hydrophilic confinement shows a larger deviation from its Gaussian form than that in the hydrophobic confinement at all temperatures, indicating that the long-time dynamics of [MMIM]+ in NaY is more heterogeneous than that in DAY. Although the orientational relaxation time scales of [MMIM]+ in the confinements significantly slowed as compared to those in bulk, confinement does not affect the librational motion of the collective hydrogen-bond network present in the IL.

Numerical Investigations on Mixing in Microchannels with Transverse Hydrophobic Strips

Typically, the flow through a microchannel network of a lab-on-a-chip device is viscous dominant. Concomitantly, the fluid transport is laminar and the intermixing of fluid layers is absent. Thus, it becomes a challenging task to rapidly homogenize different species within the limited spacial region available. In this context, to enhance the micro-mixing, we extend the work of Ranjith et al. [Soft Matter (2014), Microfluid. Nanofluid. (2015)] who suggested that a sudden drift in wall shear stress generates cross-stream momentum. Such microchannels are realized by periodically coating a hydrophilic wall with hydrophobic materials by keeping the orientation of these regions in the transverse direction to the pressure gradient. In the present study, the computational fluid dynamics is employed to investigate the convective mixing in Y-mixers decorated with such hybrid hydrophobic-hydrophilic surfaces. These microchannels are considered as a combination of several alternately repeating hydrophobic and hydrophilic patches which indeed modeled as an agglomeration of no-slip and partial-slip surfaces. Here, the improvement in mixing owing to the churning of species inside these channels in comparison with conventional hydrophilic confinement is examined in detail. We observed that, the lateral velocity generated throughout stripped channel promotes homogenization in comparison with its uniformly hydrophobic counterpart. In addition to that, the presence of hydrophobic surfaces reduces the hydrodynamic resistance offered by the channel, which in turn saves the pumping power significantly.

The behaviour of water and sodium chloride solution confined into asbestos nanotube

ABSTRACTWe present the molecular simulation study of the behaviour of water and sodium chloride solution confined in lizardite asbestos nanotube which is a typical example of hydrophilic confinement. The local structure and orientational and dynamic properties are studied. It is shown that at low enough temperatures there is a well-defined orientational ordering of the water molecules. At high local densities corresponding to the maxima of the density distribution function, the water molecules are oriented parallel to the axis of the tube. It is also shown that the diffusion coefficient drops about two orders of magnitude comparing to the bulk case. The behaviour of sodium chloride solutions is also considered and the formation of double layer is observed.

Instantaneous Directional Growth of Block Copolymer Nanowires During Heterogeneous Radical Polymerization (HRP)

Polymeric nanowires that consist of ultrahigh molecular weight block copolymers were instantaneously prepared via one-step surfactant-free heterogeneous radical polymerization (HRP). Under heterogeneous reaction and initiator-starvation conditions, the sequential copolymerization of hydrophilic and hydrophobic monomers facilitates the formation of amphiphilic ultrahigh molecular weight block copolymers, which instantaneously assemble to polymeric nanowires. As polymerization progresses, initially formed nanoparticles exhibit the directional growth due to localized repulsive forces of hydrophilic blocks and confinement of the hydrophobic blocks that adopt favorable high aspect ratio nanowire morphologies. Using one-step synthetic approach that requires only four ingredients (water as a solvent, two polymerizable monomers (one hydrophilic and one hydrophobic), and water-soluble initiator), block copolymer nanowires ∼70 nm in diameter and hundreds of microns in length are instantaneously grown. For example, when 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and styrene (St) were copolymerized, high aspect ratio nanowires consist of ultrahigh (>10(6) g/mol) molecular weight pDMAEMA-b-St block copolymers and the presence of temperature responsive pDMAEMA blocks facilitates nanowire diameter changes as a function of temperature. These morphologies may serve as structural components of the higher order biological constructs at micro and larger length scales, ranging from single strand nanowires to engineered biomolecular networks capable of responding to diverse and transient environmental signals, and capable of dimensional changes triggered by external stimuli.

Structural behavior of an anomalous fluid under hydrophobic, hydrophilic and heterogeneous confinement

We perform molecular dynamics simulations of an anomalous fluid confined between parallel plates. The effects of the confinement on the structural behavior of the fluid when the plates are hydrophobic, hydrophilic and amphiphilic were investigate. The anomalous fluid was modeled using a two length scales potential, that exhibits density, diffusion and structural anomalies like observed in liquid water. We show that this fluid at the contact layer exhibits the formation of a solid/amorphous phase for temperatures at which the bulk system is liquid.

Nano-confinement of biomolecules: Hydrophilic confinement promotes structural order and enhances mobility of water molecules

Molecular dynamics simulations have been used to investigate the confinement packing characteristics of small hydrophilic (N-acetyl-glycine-methylamide, Nagma) and hydrophobic (N-acetyl-leucine-methylamide, Nalma) biomolecules in large diameter single-wall carbon nanotubes (SWCNTs). We find that hydrophilic biomolecules easily fill the nanotube and self organize into a geometrical configuration which reminds the water structural organization under SWCNT confinement. The packing of hydrophilic biomolecules inside the cylinder confines all water molecules in its core, which enhances their mobility. Conversely, hydrophobic biomolecules accommodate into the nanotubes with a trend for homogeneous filling, which generate unstable small pockets of water and drive toward a state of dehydration. These results shed light on key parameters important for the encapsulation of biomolecules with direct relevance for long-term storage and prevention of degradation.

Density of hydrophobically confined deeply cooled water investigated by small angle X-ray scattering

Water's behavior near hydrophobic surfaces has attracted great attention due to chemical and geological applications. Here, we report small angle X-ray scattering (SAXS) studies of water confined in the hydrophobic nanoporous carbon material, CMK-1-14, from ambient to deeply cooled temperatures. By monitoring the scattering intensity of the first Bragg peak, which is directly related to the scattering length density contrast between the carbon matrix and the confined water, the average density of the hydrophobically confined water was determined from 300 K to 150 K at ambient pressure. Furthermore, differential scanning calorimetry and X-ray diffraction measurements showed that the majority of such hydrophobically confined water did not crystallize in the investigated temperature range. By exploiting the fast speed of SAXS measurements and the continuous temperature ramping, the average density profile and the deduced thermal expansion coefficient (αp) were obtained. We found that the well-known density maximum of water at 277 K downshifted to 260 K, and the density minimum which has been observed in hydrophilic confinement disappeared. In addition, the previously measured large density decreasing of 18% at low temperature was recalibrated to a more reasonable 10% instead. Consequently, the recalculated αp peak was found to be quite similar to that of the water confined in hydrophilic MCM-41-S-15 suggesting an intrinsic property of water, which does not sensitively depend on the confinement surface.

Improved Light Uniformity From Light-Emitting Diodes by Heterogeneous Microlenses and 3-D Printed Mold

We present a novel design and fabrication process using the hydrophilic confinement effect and a 3-D printer for producing convex and concave microlenses on top of light-emitting diodes. We use energy conservation and a lawnmower algorithm to design a freeform lens, and transform it into heterogeneous microlenses. The diameters of the microlenses on top of the light-emitting diode are 500 and 200 μm for the concave and convex shapes, respectively. Heterogeneous microlenses are printed on a glass substrate using the hydrophilic confinement effect of SU-8 photoresist and an inkjet printer. Then, the array is copied with a mold made by a 3-D printer for the replication process. In the replication process, the mold is filled with polydimethylsiloxane to reproduce heterogeneous microlenses. Experimental results show that this heterogeneous microlens array improves the light distribution uniformity of a light-emitting diode from 17% to 63%.