Min Pack

Bio: Min Pack is an academic researcher from Drexel University. The author has contributed to research in topics: Drop (liquid) & Air entrainment. The author has an hindex of 8, co-authored 14 publications receiving 185 citations. Previous affiliations of Min Pack include Pennsylvania State University & Princeton University.

Colloidal Drop Deposition on Porous Substrates: Competition among Particle Motion, Evaporation, and Infiltration.

Abstract: Recent interest in printable electronics and in particular paper- and textile-based electronics has fueled research in inkjet printing of colloidal drops on porous substrates. On nonporous substrates, the interplay of particle motion and solvent evaporation determines the final deposition morphology of the evaporating colloidal drop. For porous substrates, solvent infiltration into the pores adds a layer of complexity to the deposition patterns that have not been fully elucidated in the literature. In this study, the deposition of picoliter-sized aqueous colloidal droplets containing nanometer- and micrometer-sized particles onto nanoporous anodic aluminum oxide substrates is examined for different drop and particle sizes and relative humidities as well as pore diameters, porosities, and wettabilities of the porous substrates. For the cases considered, solvent infiltration is found to be much faster than both evaporation and particle motion near the contact line, and thus when the substrate fully imbibes ...

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

Abstract: 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.

Deposition of Colloidal Drops Containing Ellipsoidal Particles: Competition between Capillary and Hydrodynamic Forces

Abstract: Ellipsoidal particles have previously been shown to suppress the coffee-ring effect in millimeter-sized colloidal droplets Compared to their spherical counterparts, ellipsoidal particles experience stronger adsorption energy to the drop surface where the anisotropy-induced deformation of the liquid–air interface leads to much greater capillary attractions between particles Using inkjet-printed colloidal drops of varying drop size, particle concentration, and particle aspect ratio, the present work demonstrates how the suppression of the coffee ring is not only a function of particle anisotropy but rather a competition between the propensity for particles to assemble at the drop surface via capillary interactions and the evaporation-driven particle motion to the contact line For ellipsoidal particles on the drop surface, the capillary force (Fγ) increases with the particle concentration and aspect ratio, and the hydrodynamic force (Fμ) increases with the particle aspect ratio but decreases with drop siz

Failure mechanisms of air entrainment in drop impact on lubricated surfaces

Abstract: Lubricated surfaces have recently been introduced and studied due to their potential benefit in various configurations and applications. Combining the techniques of total internal reflection microscopy and reflection interference microscopy, we examine the dynamics of an underlying air film upon drop impact on a lubricated substrate where the thin liquid film is immiscible to the drop. In contrast to drop impact on solid surfaces where even the smallest asperities cause random breakup of the entraining air film, we report two air film failure mechanisms on lubricated surfaces. In particular, using ≈5 μm thick liquid films of high viscosity, which should make the substrate nearly atomically smooth, we show that air film rupture shifts from asperity-driven to a controlled event. At low Weber numbers (We 10), the air film failure occurs much earlier in time at the first inflection point of the air film shape away from the drop center, where the liquid–liquid van der Waals interactions become important. The predictable failure modes of the air film upon drop impact sheds light on droplet deposition in applications such as lubricant-infused self-cleaning surfaces.

Design of Affordable Greenhouses for East Africa

Abstract: Reflecting the severity of global food insecurity, over 60% of the East African population is considered malnourished, with many regions in a state of famine. There is broad agreement on the need to help small-scale farmers move from subsistence to sustainable and profitable farming by boosting their agricultural productivity, reducing post-harvest spoilage losses and providing market linkages. Most countries in East Africa have an agrarian economy with over 80% of the households depending on agriculture for their livelihoods. The climate is characterized by biannual dry seasons where many farmers suffer due to water shortages and poor soil nutrition. While short periods of rain benefit local farmers, heavy rainfall sometimes destroys cash crops. Greenhouses are permanent glass or plastic-covered structures that allow farmers to grow vegetables and fruits year-round through mechanically-controlled temperature and irrigation systems. Greenhouses can help farmers in East Africa grow and protect crops in both wet and dry seasons. Large commercial farms employ large greenhouses to produce high-value cash crops for the export market. East African companies import and sell greenhouses priced at over US $2,000 to commercial farmers. While greenhouses can significantly increase smallholder productivity and improve livelihoods, current designs are inappropriate and too expensive. The adoption of affordable and context-appropriate greenhouses can lead to improved livelihoods for farmers and entrepreneurs while fostering food security. This paper describes the constraints and design tenets for low-cost (~$200 bill of materials) greenhouses and discusses results from three years of field-testing such greenhouses in Kenya, Tanzania and the United States. Currently, the field-tested prototypes excel in affordability, maintenance, and crop protection. This paper seeks collaborators to refine and localize the technology with the ultimate objective of disseminating it broadly.

Dewetting of Thin Polymer Films

Abstract: Abstract.We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process.

Fundamental Fluid Dynamics Challenges in Inkjet Printing

Abstract: Inkjet printing is the most widespread technological application of microfluidics. It is characterized by its high drop productivity, small volumes, and extreme reproducibility. This review gives a synopsis of the fluid dynamics of inkjet printing and discusses the main challenges for present and future research. These lie both on the printhead side—namely, the detailed flow inside the printhead, entrained bubbles, the meniscus dynamics, wetting phenomena at the nozzle plate, and jet formation—and on the receiving substrate side—namely, droplet impact, merging, wetting of the substrate, droplet evaporation, and drying. In most cases the droplets are multicomponent, displaying rich physicochemical hydrodynamic phenomena. The challenges on the printhead side and on the receiving substrate side are interwoven, as optimizing the process and the materials with respect to either side alone is not enough: As the same ink (or other jetted liquid) is used and as droplet frequency and size matter on both sides, the process must be optimized as a whole.