Nate Cira

Vapour-mediated sensing and motility in two-component droplets

Controlling the wetting behaviour of liquids on surfaces is important for a variety of industrial applications such as water-repellent coatings and lubrication. Liquid behaviour on a surface can range from complete spreading, as in the ‘tears of wine’ effect, to minimal wetting as observed on a superhydrophobic lotus leaf. Controlling droplet movement is important in microfluidic liquid handling, on self-cleaning surfaces and in heat transfer. Droplet motion can be achieved by gradients of surface energy. However, existing techniques require either a large gradient or a carefully prepared surface to overcome the effects of contact line pinning, which usually limit droplet motion. Here we show that two-component droplets of well-chosen miscible liquids such as propylene glycol and water deposited on clean glass are not subject to pinning and cause the motion of neighbouring droplets over a distance. Unlike the canonical predictions for these liquids on a high-energy surface, these droplets do not spread completely but exhibit an apparent contact angle. We demonstrate experimentally and analytically that these droplets are stabilized by evaporation-induced surface tension gradients and that they move in response to the vapour emitted by neighbouring droplets. Our fundamental understanding of this robust system enabled us to construct a wide variety of autonomous fluidic machines out of everyday materials.

Trap it!: A Playful Human-Biology Interaction for a Museum Installation

We developed Trap it!, a human-biology interaction (HBI) medium encompassing a touchscreen interface, microscopy, and light projection. Users can interact with living cells by drawing on a touchscreen displaying the microscope view of the cells. These drawings are projected onto the microscopy field as light patterns, prompting observable movement in phototactic responses. The system design enables stable and robust HBI and a wide variety of programmed activities (art, games, and experiments). We investigated its affordances as an exhibit in a science museum in both facilitated and unfacilitated contexts. Overall, it had a low barrier of entry and fostered rich communication among visitors. Visitors were particularly excited upon realizing that the interaction involved real organisms, an understanding that was facilitated by the eyepiece on the physical system. With the results from user study, we provide our observations, insights and guidelines for designing HBI as a permanent museum exhibit.

Rapid Identification of ESKAPE Bacterial Strains Using an Autonomous Microfluidic Device

This article describes Bacteria ID Chips (‘BacChips’): an inexpensive, portable, and autonomous microfluidic platform for identifying pathogenic strains of bacteria. BacChips consist of a set of microchambers and channels molded in the elastomeric polymer, poly(dimethylsiloxane) (PDMS). Each microchamber is preloaded with mono-, di-, or trisaccharides and dried. Pressing the layer of PDMS into contact with a glass coverslip forms the device; the footprint of the device in this article is ∼6 cm2. After assembly, BacChips are degased under large negative pressure and are stored in vacuum-sealed plastic bags. To use the device, the bag is opened, a sample containing bacteria is introduced at the inlet of the device, and the degased PDMS draws the sample into the central channel and chambers. After the liquid at the inlet is consumed, air is drawn into the BacChip via the inlet and provides a physical barrier that separates the liquid samples in adjacent microchambers. A pH indicator is admixed with the samples prior to their loading, enabling the metabolism of the dissolved saccharides in the microchambers to be visualized. Importantly, BacChips operate without external equipment or instruments. By visually detecting the growth of bacteria using ambient light after ∼4 h, we demonstrate that BacChips with ten microchambers containing different saccharides can reproducibly detect the ESKAPE panel of pathogens, including strains of: Enterococcus faecalis, Enteroccocus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter aerogenes, and Enterobacter cloacae. This article describes a BacChip for point-of-care detection of ESKAPE pathogens and a starting point for designing multiplexed assays that identify bacterial strains from clinical samples and simultaneously determine their susceptibility to antibiotics.

LudusScope: Accessible Interactive Smartphone Microscopy for Life-Science Education

For centuries, observational microscopy has greatly facilitated biology education, but we still cannot easily and playfully interact with the microscopic world we see. We therefore developed the LudusScope, an accessible, interactive do-it-yourself smartphone microscopy platform that promotes exploratory stimulation and observation of microscopic organisms, in a design that combines the educational modalities of build, play, and inquire. The LudusScope’s touchscreen and joystick allow the selection and stimulation of phototactic microorganisms such as Euglena gracilis with light. Organismal behavior is tracked and displayed in real time, enabling open and structured game play as well as scientific inquiry via quantitative experimentation. Furthermore, we used the Scratch programming language to incorporate biophysical modeling. This platform is designed as an accessible, low-cost educational kit for easy construction and expansion. User testing with both teachers and students demonstrates the educational potential of the LudusScope, and we anticipate additional synergy with the maker movement. Transforming observational microscopy into an interactive experience will make microbiology more tangible to society, and effectively support the interdisciplinary learning required by the Next Generation Science Standards.

Dancing droplets: Autonomous surface tension-driven droplet motion

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Dancing droplets: Autonomous surface tension-driven droplet motion Nate J. Cira, Adrien Benusiglio, Manu Prakash

Neutral and niche dynamics in a synthetic microbial community

Ecologists debate the relative importance of niche versus neutral processes in understanding biodiversity1,2. This debate is especially pertinent to microbial communities, which play crucial roles in biogeochemical cycling3,4, food production5, industrial processes6,7, and human health and disease8. Here we created a synthetic microbial community using heritable genetic barcodes and tracked community composition over time across a range of experimental conditions. We show that a transition exists between the neutral and niche regimes, and, consistent with theory, the crossover point depends on factors including immigration, fitness, and population size. We find that diversity declined most rapidly at intermediate population sizes, which can be explained by a tradeoff between replacement by migration and duration of growth. We then ran an experiment where the community underwent abrupt or gradual changes in size, the outcome of which highlights that selecting the correct model is essential to managing diversity. Taken together these results emphasize the importance of including niche effects to obtain realistic models across a wide range of parameters, even in simple systems.

Correction: LudusScope: Accessible Interactive Smartphone Microscopy for Life-Science Education

[This corrects the article DOI: 10.1371/journal.pone.0162602.].

Neutral and selective dynamics in a synthetic microbial community

Significance We created a synthetic microbial community to help understand how evolution and selection pressure change the species diversity of an ecosystem. Our results show that there is a clear transition between neutral and selective regimes that depends on the rate of immigration as well as the fitness differences. Ecologists debate the relative importance of selective vs. neutral processes in understanding biodiversity. This debate is especially pertinent to microbial communities, which play crucial roles in areas such as health, disease, industry, and the environment. Here, we created a synthetic microbial community using heritable genetic barcodes and tracked community composition over repeated rounds of subculture with immigration. Consistent with theory, we find a transition exists between neutral and selective regimes, and the crossover point depends on the fraction of immigrants and the magnitude of fitness differences. Neutral models predict an increase in diversity with increased carrying capacity, while our selective model predicts a decrease in diversity. The community here lost diversity with an increase in carrying capacity, highlighting that using the correct model is essential for predicting community response to change. Together, these results emphasize the importance of including selection to obtain realistic models of even simple systems.