Research Article www.acsami.org
Adhesion of Microdroplets on Water-Repellent Surfaces toward the Prevention of Surface Fouling and Pathogen Spreading by Respiratory Droplets Jieke Jiang,†,‡ Hengdi Zhang,† Wenqing He,† Tianzhong Li,† Hualin Li,† Peng Liu,† Meijin Liu,† Zhaoyue Wang,† Zuankai Wang,§ and Xi Yao*,† †
Department of Biomedical Sciences, ‡School of Veterinary Medicine, and §Department of Mechanical and Biomedical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China S Supporting Information *
ABSTRACT: Biofouling caused by the adhesion of respiratory microdroplets generated in sneezing and coughing plays an important role in the spread of many infectious diseases. Although water-repellent surfaces are widely used for the long-term repellency of aqueous solutions, their repellency to pathogen-containing microdroplets is elusive. In this work, microdroplets from picoliter to nanoliter were successfully generated in a controlled manner to mimic the exhaled microdroplets in sneezing and coughing, which allowed us to evaluate the adhesion of microdroplets on both superhydrophobic and lubricant-infused “slippery” surfaces for the first time. The impact and retention of water microdroplets on the two water-repellent surfaces are compared and investigated. Microdroplet-mediated surface biofouling and pathogen transmission were also demonstrated. Our results suggested that the adhesion of microdroplets should be duly considered in the design and application of water-repellent surfaces on biofouling prevention. KEYWORDS: microdroplets, aerosol, superhydrophobic, lubricant-infused, antibiofouling
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of their repellency to aqueous solution.13−17 Particularly, the lotus-leaf-inspired superhydrophobic (SH) and the pitcherplant-inspired lubricant-infused “slippery” (LIS) surfaces are two representative examples.18,19 The SH surfaces can trap air inside the surface textures to reduce the contact area of the water−solid interface and therefore achieve a contact angle larger than 150° and a sliding angle lower than 10°.20−22 Researchers have utilized the water-repellent SH surfaces for antibiofouling applications based on the idea that the biofluids or complex fluids cannot stay on the SH surface long enough for a conditioning layer of biomolecules or microorganism to form.23−25 Different from the SH surface, the LIS surface utilized an overcoated, immiscible lubricant layer to protect the underneath solid substrate and to achieve sliding angles lower than 5° for liquids that are immiscible to the lubricant.26−31 Exemplified biofluids on which biofouling could be prevented on the LIS surfaces include blood,32 proteins, 33 and bacteria.34,35 However, those studies on the antibiofouling performance of both SH and LIS surfaces are merely based on an evaluation with a bulk medium with volume larger than 1 μL, which is different from the respiratory microdroplets (with
INTRODUCTION Violent respiratory events such as coughing and sneezing play a key role in transferring many infectious diseases. Those respiratory activities can generate numerous tiny droplets.1,2 For example, more than 3000 tiny droplets could be generated by a sneeze, while a cough produces almost 40000 tiny droplets. Most of the droplets have diameters of less than 100 μm, which makes them float in the air and contaminate elsewhere.3 Those microdroplets are believed to mediate the transmission and spread of many infectious diseases between individuals and in hospitals because they provide a vector of pathogens that contributes to the transmission of disease in different modes.4 For example, surfaces of public facilities are easily fouled by respiratory microdroplets and the adhered pathogens could be viral in several hours.5 Those pathogens could be transmitted from the contaminated surfaces to human hands and then to the mucous membranes of eyes or mouth via contact transmission.6−9 During the fouling process, one significant step is the adhesion and retention of microdroplets on the surface. Therefore, reducing the adhesion of microdroplets on surfaces is promising for reducing microdroplet-mediated surface fouling and for controlling the transmission of respiratory diseases.10−12 Bioinspired water-repellent surfaces have offered great opportunities for real-world applications of antifouling because © XXXX American Chemical Society
Received: November 27, 2016 Accepted: January 25, 2017 Published: January 25, 2017 A
DOI: 10.1021/acsami.6b15213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Generation and measurement of aerosolized microdroplets in the spraying method. (a) Photograph capturing a sneeze in progress. Numerous microdroplets were expelled as the sneeze happened. (b) Photograph capturing the spraying process of the sprayer used to mimic the respiratory activity. (c) SEM images of NaCl microcrystals on a SH surface. NaCl crystals were formed after the sprayed microdroplets of a NaCl solution evaporated on the surface. (d) Distribution of the NaCl crystal edge length at the collecting position with a 15 cm distance to the nozzle. The edge length of the NaCl crystal was used to calculate the size of the sprayed microdroplets. (e) Volume distribution of microdroplets generated by the sprayer. Part a is adopted from the CDC Web site with ID 11162, and this image is free of copyright restriction. Photo credit: James Gathany.
volume less than 1 nL). Moreover, retention of microdroplets at a nanoliter scale or smaller has indeed been observed on both SH and LIS surfaces in the situations of low-temperature condensation and icing,36−39 but these do not fit the respiratory activities that happen in mild conditions. The direct observation and evaluation of microdroplet adhesion on typical water-repellent surfaces remains elusive, and the correlation between the potential microdroplet adhesion on waterrepellent surfaces and pathogen spreading is not clear, which is essential in controlling the contact transmission of infectious diseases. To study the abilities of different water-repellent surfaces in preventing microdroplet-mediated biofouling, nanostructured SH and gel-based LIS surfaces were first prepared, on which the retention of aerosolized microdroplets containing bacteria was investigated. The size of the aerosolized microdroplets was controlled to mimic the respiratory microdroplets. The retention and transmission of the microdroplet-mediated bacteria were evaluated based on fluorescent imaging and culture methods, which highlighted the crucial role of microdroplet adhesion in the process of pathogen transmission. Furthermore, individual microdroplets were generated by a SH needle, and their impact on water-repellent surfaces was studied. The adhesion mechanisms of microdroplets on the two surfaces and the resulting bacterial transmission were
compared. Our results showed that the nanostructured SH surface has a better performance in reducing microdropletmediated pathogen transmission than the gel-based LIS surface. Unlike the larger droplets that could slide away on the LIS surface, microdroplets can more easily be trapped by the lubricant layer on the LIS surface, which causes further biofouling.
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RESULTS AND DISCUSSION
To mimic the microdroplets produced in real respiratory activities (Figure 1a shows a sneeze generating numerous microdroplets), a sprayer with controlled air flow was used to aerosolize fluids to generate microdroplets (Figure 1b). The diameters of the sprayed microdroplets were measured through an indirect method. Briefly, the aerosolized microdroplets were sprayed from a NaCl solution and then collected by a SH substrate at different positions from the nozzle. After evaporation, cubic NaCl microcrystals formed on the SH substrate (Figures 1c and S1), the size of which was then measured by scanning electron microscopy (SEM; Figure 1d). Therefore, the size distribution of the microdroplets was calculated based on the size distribution of the NaCl crystals and the concentration of the NaCl solution. Basically, the spraying was controlled by adjusting the pressure of the input air flow, and the distribution of the microdroplets was adjusted B
DOI: 10.1021/acsami.6b15213 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Parameters of the SH and LIS Surfacesa pitch, p (μm) SH surface LIS surface
diameter, d (μm)
0.74 + 0.22 0.35 + 0.07 thickness of the lubricant (μm)
solid fraction, ΦS
roughness,R
contact angle (deg)
sliding angle (deg)
23.85 157 + 2 surface tension of the lubricant (mN/m)
0.18 contact angle (deg)
1.5 sliding angle (deg)
20
93 + 3
