S2E2

Abstract:

4 billion people in the world are facing severe water scarcity[1]. To address this problem, mainstream research has been focused on desalination of brine[2], water harvesting from air[3], etc. However, the effort to reduce water consumption in daily life has not received enough attention. For example, more than 141 million m3 of fresh water is flushed away in toilets every day globally[4], nearly 6 times as much as the daily water consumption of the entire Africa population[5]. To reduce the flushing water, toilet surfaces need to repel sticky viscoelastic solid (e.g. human fecal waste). Here, we create a design criterion for viscoelastic solid repellent coatings, and develop a facile, scalable coating method for materials in toilets, which significantly reduces the adhesion of the human fecal waste and saves considerable amount of flushing water. Specifically, we demonstrate that our designed liquid-entrenched smooth surfaces (LESS) are only 10% of the adhesion of untreated smooth surfaces to synthetic human feces, and saves up to 90% of water used for cleaning untreated surfaces. In addition, LESS show great anti-bacteria performance, even better comparing to slippery liquid-infused porous surfaces (SLIPS)[6] with underlying surface roughness. With all the function of LESS, we provide a lubricant replenishment strategy to address the concern of coating longevity, and we demonstrate its effectiveness on maintaining the coating’s functionality from both physics modeling and real testing. We believe that the slippery coating in this study provides new possibilities in solving small problems in daily life; while contributing globally to the mitigation of the water scarcity, as well as improving quality of life in developing regions.

Reference

[1] M. M. Mekonnen and A. Y. Hoekstra, "Four billion people facing severe water scarcity," Science Advances, vol. 2, 2016.

[2] M. Elimelech and W. A. Phillip, "The Future of Seawater Desalination: Energy, Technology, and the Environment," Science, vol. 333, pp. 712-717, Aug 2011.

[3] H. Kim, S. Yang, S. R. Rao, S. Narayanan, E. A. Kapustin, H. Furukawa, et al., "Water harvesting from air with metal-organic frameworks powered by natural sunlight," Science, 2017.

[4] W. H. Organization and UNICEF, Progress on sanitation and drinking water–2015 update and MDG assessment: World Health Organization, 2015.

[5] U. N. D. Programme, Human Development Report: UNDP, 2006.

[6] T. S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A. Grinthal, et al., "Bioinspired Self-repairing Slippery Surfaces with Pressure-stable Omniphobicity," Nature, vol. 477, p. 443, 2011.

Abstract:

Polymer interfaces play a critical role in a variety of applications, including consumer products, structural components, biomedical devices, and electronics. In many cases, polymers need to be bonded with adhesives to create structural joints or multi-layer structures. For adhesives to efficiently wet and bond to a substrate, the surface free energy of the substrate must be equal to or higher than the surface free energy of the adhesive. However, the surface energy of most polymers is low, which makes adhesion difficult. Thus, there often is a need to increase the surface energy of a polymer without changing the bulk mechanical and chemical properties.

In this work, we demonstrate that atomic layer deposition (ALD) can be applied on poly(methyl methacrylate) (PMMA) and fluorinated ethylene propylene (FEP) to increase their surface energies and, hence, to increase the interfacial fracture toughness when bonded to an epoxy adhesive.

ALD alumina films were deposited on each type of polymer to modify the surfaces towards high energy surfaces of metal oxide. Transmission-electron microscopy (TEM) and atomic-force microscopy (AFM) were used to study the film morphology on the polymers. These indicated that the ALD treatment increased the surface roughness and changed the sub-surface chemistry by vapor-phase infiltration (VPI). The increase in surface energy after ALD was measured by the sessile-drop test with water, ethylene glycol and glycerol.

The interfacial fracture toughness of each polymer-epoxy interface was measured using a customized motor-controlled wedge tester. After ALD film growth, the interfacial fracture toughness of the PMMA-epoxy and FEP-epoxy interfaces increased by factors of up to 7 and 60, respectively. The two ALD samples and two control samples were tested at the same level of humidity. Furthermore, we observed stress-corrosion cracking of the ALD-polymer interfaces. By conducting wedge tests in different levels of humidity, we found that although ALD increased equilibrium interfacial fracture toughness at all humidity, the effect decreased as humidity increased. Scanning-electron microscopy (SEM) of samples after testing provided additional evidence for stress-corrosion cracking of the ALD-polymer interface. These results suggest a new application of ALD for engineering the mechanical properties of chemically inert surfaces.

Abstract:

Grippers are widely used for the gripping, manipulation, and assembly of objects with a wide range of scales, shapes, and quantities in research, industry, and our daily lives. A simple yet universal solution is very challenging. Here, we manage to address this challenge utilizing a simple shape memory polymer (SMP) block. The embedding of objects into the SMP enables the gripping while the shape recovery upon stimulation facilitates the releasing. Systematic studies show that friction, suction, and interlocking effects dominate the grip force individually or collectively. This universal SMP gripper design provides a versatile solution to grip and manipulate multi-scaled (from centimeter scale down to 10-um scale) 3D objects with arbitrary shapes, in individual, deterministic, or massive, selective ways. These extraordinary capabilities are demonstrated by the gripping and manipulation of macro-scaled objects, meso-scaled steel sphere arrays and microparticles, and the selective and patterned transfer printing of micro light-emitting diodes.