Cheng Luo

Branched ZnO Wire Structures for Water Collection Inspired by Cacti

In this work, motivated by an approach used in a cactus to collect fog, we have developed an artificial water-collection structure. This structure includes a large ZnO wire and an array of small ZnO wires that are branched on the large wire. All these wires have conical shapes, whose diameters gradually increase from the tip to the root of a wire. Accordingly, a water drop that is condensed on the tip of each wire is driven to the root by a capillary force induced by this diameter gradient. The lengths of stem and branched wires in the synthesized structures are in the orders of 1 mm and 100 μm, respectively. These dimensions are, respectively, comparable to and larger than their counterparts in the case of a cactus. Two groups of tests were conducted at relative humidity of 100% to compare the amounts of water collected by artificial and cactus structures within specific time durations of 2 and 35 s, respectively. The amount of water collected by either type of structures was in the order of 0.01 μL. However, on average, what has been collected by the artificial structures was 1.4-5.0 times more than that harvested by the cactus ones. We further examined the mechanism that a cactus used to absorb a collected water drop into its stem. On the basis of the gained understanding, we developed a setup to successfully collect about 6 μL of water within 30 min.

Fibroblast/fibrocyte: surface interaction dictates tissue reactions to micropillar implants.

Micropillar technology has shown great promise for medical implants or sensors in recent years. To study the influence of surface topography on cellular responses, polydimethylsiloxane (PDMS) micropillar arrays with pillar spacing (20-70 μm) and height (14-25 μm) have been fabricated. The influence of micropillar arrays on cellular behavior was tested both in vitro and in vivo. Interestingly, in vitro, we observe a distinct response for 3T3 fibroblasts and RAW 264.7 macrophages to the topographical cues tested. Attachment and proliferation of fibroblasts was substantially enhanced by increasing pillar height, whereas macrophage adherence is significantly diminished by reduced pillar spacing. When implanted in the subcutaneous cavity of BALB/c mice for 14 days, we find a prevailing trend with capsule cell density and capsule thickness increasing, as both pillar height and spacing rise. Collagen deposition and neoangiogenesis, two pivotal factors in granulation tissue maturation, are also observed to have a stronger response to the increase in both pillar height and spacing. In contradiction to our original hypothesis, we observed that fibroblasts rather than macrophages are a key contributor to the in vivo outcome of micropillar arrays. Investigation into fibroblast activation, however, revealed that recruited fibrocytes, rather than resident fibroblasts, correspond to the in vivo outcome. The results from this work support the critical and often overlooked role of fibrocytes in tissue response to biomaterial implants with varying topography.

Propulsion of microboats using isopropyl alcohol as a propellant

In this work, we explored the possibility to develop a microboat for potentially transporting desired targets in microfluidic systems. We studied its design, fabrication, actuation and motions. In three types of tests conducted on water surfaces of different heights in a 30-cm-long channel, the microboat had a speed in the order of 0.1 m s−1, and propulsive forces ranged from 159 to 250 µN. It took 1–2 s for the microboat to go through the channel, and resistance coefficients varied from 0.144 to 0.324. Also, when the resistance coefficient was 0.324, the microboat was still able to travel a distance of 91.4 cm within 5.33 s in a 94.5-cm-long channel.

Fabrication of super-hydrophobic channels

A new approach was developed in this work to create channels which had not only super-hydrophobic bottom surfaces but also super-hydrophobic sidewalls. Researchers have demonstrated that a flow experienced less drag forces and thus required smaller driving pressure in a channel of micro/nanostructure-formed top and bottom surfaces. The drag forces should be further reduced in a channel which has not only patterned top and bottom surfaces but also patterned sidewall surfaces. However, due to the limitation of the existing lithographic approaches, sidewalls could not be properly patterned. Therefore, a new approach was developed in this work to overcome this obstacle. Polydimethylsiloxane (PDMS) micropillars of aspect ratios 1.4, 2.0 and 2.7 were first generated on PDMS films using a molding method, and then transferred to the sidewalls and bottom surfaces of three 1 mm wide and 1 mm deep channels, respectively, applying a hot-embossing process. The corresponding deformation mechanism was considered. The widths of the PDMS films had a critical effect on the cross-section profiles of the generated channels. The radii of the channel corners and inclined degrees of the sidewalls increased with the film widths. Contact angles on the PDMS films before and after the deformations were measured and compared. The contact angles in the middle portions of the sidewalls, as well as at the bottoms of the generated channels, were little difference from those on the original PDMS films due to the small changes in the distances between the micropillars. However, the contact angles were increased and decreased, respectively, at the bottom and top corners of the generated channels since the PDMS films were compressed and stretched at these corners during the fabrication. The variation of the contact angle in each channel was further analyzed according to two existing theoretical formulas. These variations increased with the increasing aspect ratios of the PDMS micropillars. The super-hydrophobic channels fabricated could be potentially employed to reduce drag forces in microfluidic applications.

Reinforcement of PDMS masters using SU-8 truss structures

In this work, a new method was developed to increase the stiffness of polydimethylsiloxane (PDMS) masters using SU-8 truss structures, aimed at reducing residual deformations of the PDMS masters induced in the molding process. Using this method, both global and local residual deformations in the released PDMS master have been reduced.

Behavior of a liquid drop between two nonparallel plates.

Liquid drops have shown interesting behaviors between two nonparallel plates. These plates may be fixed or movable relative to each other. In this work, we also explore these behaviors through a combination of theoretical and experimental investigations and obtain some new results. We show that when the two plates are fixed, different from the previous understanding, a lyophilic drop may not necessarily fill the corner of the two plates. We also demonstrate that it may fill the corner, when more liquid is added to the drop or when the top plate is lifted. Furthermore, we propose a physical model to interpret the shifting effect of a liquid drop. This effect appears when the drop is squeezed and relaxed between two nonparallel plates, and it has been used by some shorebirds to transport prey. On the basis of the proposed model, we have found three new phenomena related to the shifting effect.

A Stable Intermediate Wetting State after a Water Drop Contacts the Bottom of a Microchannel or Is Placed on a Single Corner

It is considered that, after a water drop contacts the base of a roughness groove, water should immediately fill this roughness groove. Subsequently, Cassie-Baxter wetting state is transited to that of Wenzel. Accordingly, one of the criteria used to judge the transition from Cassie-Baxter to Wenzel states is whether a water drop has contact with the base of a roughness groove. In this work, through theoretical and experimental investigations, we show that this transition criterion does not always hold true in the case of microchannels. We first theoretically prove that, when an angle criterion is satisfied, there may exist an intermediate wetting state inside a microchannel after a water drop contacts the bottom of the microchannel. In this wetting state, water does not completely fill the microchannel, and air pockets still exist in its bottom corners. Also, the wetting state is stable in the sense that its energy state is lower than that of the Wenzel model. According to the angle criterion, such intermediate states may exist, for example, in microchannels with vertical sidewalls, when contact angles on the inner surfaces of these microchannels are larger than 135°. In addition to microchannels, the aforementioned intermediate state may also exist on a single corner (which is formed by a horizontal plate and an inclined plate), when the angle criterion is met. After theoretical modeling, we then conduct four types of tests on single corners and microchannels to validate the angle criterion. In these tests, once the angle criterion is met, stable intermediate states are observed on the corresponding samples. In addition, it is found from the two types of tests conducted on microchannels that, once Laplace pressure inside a water drop is gradually reduced, such an intermediate wetting state may be transited back to the original Cassie-Baxter state. On the other hand, the Wenzel state may not have such a reversal transition unless an additional force is applied to overcome energy barrier between Wenzel and Cassie-Baxter states.

Dry release of polymer structures with anti-sticking layer

A dry release method using a thin Teflon™ layer for SU-8 multilayered polymeric microstructures is presented. The low surface energy of Teflon makes the adhesion of SU-8 and substrate poor, enabling the SU-8 polymer photoresist to be removed after the devices have been fully processed. The surface energy was measured using the open-crack method, and the surface roughness and deformation of the released SU-8 were minimized in our processing. The dry release technique eliminates the diffusion limited problem in wet etching and is suitable to package complex three-dimensional polymer microfluidic devices. One such example, which provided the original impetus to formulate a dry release process, is a multilayered SU-8 structure that encapsulates small quantities of fluid. This device is being developed for a biomedical application, and will be used throughout this article as an example of a complex SU-8 structure that uses the dry release process.

Existence and role of large micropillars on the leaf surfaces of The President lotus.

It is reported that a lotus surface has hybrid micro/nanostructures (i.e., small micropillars are covered with nanopillars), which make a water drop easily roll off from the lotus surface. However, we have recently found that, in addition to nanopillars and small micropillars, there also exist sparsely distributed large micropillars on the leaf surface of The President lotus. Accordingly, in this work, we examined the effects of these large micropillars on the wetting properties of The President through four types of wetting experiments: pressing tests, measurement of tilt and contact angles, condensation, and evaporation. For the purpose of comparison, we also did the same experiments on the leaf surfaces of another two lotuses, Carolina Queen and Chawan Basu, which only have hybrid micro/nanostructures. The President, Carolina Queen, and Chawan Basu are three different lotus varieties.

A simple deflection-testing method to determine Poisson's ratio for MEMS applications

A method to determine Poissons ratio of thin films employing a simple experimental deflection test and a simulation program has been developed. According to the method, knowing Youngs modulus and measuring the deflection at an arbitrary point on the thin film, the Poissons ratio can be determined using any mechanical finite element analysis (FEA) program. Two examples are considered to test the method, with both yielding the same results. Due to the relative simplicity involved with this procedure, it is believed that the method can be used by MEMS researchers to determine Poissons ratios of their own thin films of interest. In addition, in the case that Poissons ratio is known and Youngs modulus is unknown, the method may be applied to find the Youngs modulus.