Interplay of Drop Shedding Mechanisms on High Wettability Contrast Biphilic Stripe-Patterned Surfaces.

To improve the rate of DWC, numerous studies have adjusted the distribution of drops through biphilic surface patterning and wettability gradients to control the nucleation and drop shedding rates on the condensing surface, yet the connection between drop shedding mechanisms and surface wettability patterning remains unclear. Moreover, wettability patterning places geometric bounds on the governing forces (i.e., gravity, capillary, and inertia), which drive the droplet shedding mechanisms. Thus, the subsequent influence of droplet distribution along the DWC regions on the shedding mechanisms may not be known a priori. In this study, the area fraction, ADWC, of the DWC and also the DWC region width, LN, were varied between 10 and 50% and 0.5-1.5 mm, respectively, to probe the dominant droplet shedding mechanisms on a high wettability contrast surface (i.e., the contact angle on the DWC was 159 ± 3.4° and the hysteresis 9 ± 3.6°, whereas the FWC was nearly perfectly wetting). Humid air was introduced inside a custom-built chamber with the upright steady-state condensation imaged by both real-time and high-speed imaging techniques. We found that the droplet shedding mechanisms changed with increasing LN where the sliding drop radii are reduced with LN while the jumping drop radii remained unchanged with LN. The maximum drop size for shedding also decreased by 13%, which we attribute to the secondary droplet inertia, which helps gravity overcome the capillary retention force. Lastly, although many studies have probed DWC enhancements via surface wettability patterning, an optimal combination of ADWC and LN provided in this study significantly aids in the improvement of future DWC-based condensers and water collector applications.

Oscillation Dynamics of Drops on Immiscible Thin Liquid Films.

While drop oscillation dynamics has been widely studied for many decades, the influence of a moving contact line on the oscillation modes of drops remains underexplored. Herein, we report the oscillation dynamics of drops on thin liquid films with different viscosities where lower viscosities provide a slipping surface and higher viscosities immobilize the contact line. A gently deposited drop onto an oil film undergoes shape oscillations due to capillarity, where the frequency, amplitude, and apparent contact angle are tracked via a high-speed camera. This study demonstrates that restraining the mobility of the drop contact line by increasing the viscosity of a thin oil film underneath the drop increases the extent of the drop oscillation time as well as affecting the natural frequency of the drop oscillation. The drop oscillation time was defined by the time at which the changes in the drop height dropped to values less than 1% of the equilibrium height. The experimental results for the first longitudinal mode oscillation frequencies as a function of the equilibrium contact angles for the pinning and slipping contact lines were in good agreement with previously reported numerical simulations and model predictions.

Drop Bouncing Dynamics on Ultrathin Films.

For drops to contact various surfaces, the removal of the interstitial fluid is the prerequisite to contact. While the conventional understanding is for drops to irreversibly spread on a film made of the same substance, we describe the dynamics of drops initiating contact yet carrying enough momentum to completely lift off of the substrate which we label as contact bouncing. We report new experimental results of the dynamics between drops impacting thin films described by the ratio of the liquid film hL to the drop with diameter D0 for the range of 0.004 < hL/D0 < 0.08. Using high-speed interferometry, we visualize the interfacial gas layer spatiotemporal signatures across the various film thicknesses and Weber numbers. We find that while increasing the deformability of the thin films enhances the gas entrainment phenomenon at early times, it also increases the rate of the gas purging rate, increasing the chance of contact just prior to the gas film retraction and drop lift off sequence. Drops which contact the liquid film during the retraction stage are able to bounce with <5% volume loss.

4D printing of self-folding and cell-encapsulating 3D microstructures as scaffolds for tissue-engineering applications.

Technology of tissue-engineering advanced rapidly in the last decade and motivated numerous studies in cell-engineering and biofabrication. Three-dimensional (3D) tissue-engineering scaffolds play a critical role in this field, as the scaffolds provide the biomimetic microenvironments that could stimulate desired cell behaviors for regeneration. However, despite many achievements, the fabrication of 3D scaffold remains challenging due to the difficulty of encapsulating cells in 3D scaffolds, controlling cell-cell organization in 3D, and being adapted by users unfamiliar with 3D biofabrication. In this study, we circumvent these obstacles by creating a four-dimensional (4D) inkjet-printing platform. This platform produces micropatterns that self-fold into a 3D scaffold. Seeding live cells uniformly onto the micropatterns before self-folding leads to cell-encapsulating 3D scaffolds with layer-wise cell-cell organization. Photo-crosslinkable biomaterial-inks of distinct swelling rates were synthesized from gelatin, and the biomaterial-inks were patterned by a customized high-precision inkjet-printer into bilayer micropatterns that were capable of self-folding into 3D microstructures. A mathematical model was developed to help design self-folding and to aid the understanding of the self-folding mechanism. Human umbilical vein endothelial cells (HUVECs) were embedded in self-folded microtubes to mimic microvessels. HUVECs in the microtube spread, proliferated, showed high cell viability, and engrafted on the microtube's inner wall mimicking the native endothelial cells. For physician and biologist end-users, this 4D printing method provides an easy-to-use platform that supports standard two-dimensional cell-seeding protocol while enabling the users to customize 3D cellularized scaffold as desired. This work demonstrated 4D printing as a promising tool for tissue-engineering applications.