Background on Research

     Vorticella convallaria is a species of ciliates that is usually found in freshwater habitats, and it has two forms: the sessile form and the swimming form. In the sessile form, Vorticella is shaped somewhat like a balloon because it has a body (the head of the balloon; about 40 μm in size) with cilia and a stalk (the string of a balloon) anchoring the cell body to a solid surface. In the swimming form, Vorticella does not have the stalk, and it uses the cilia to swim around in water. Because the cilia are located on only one end of the cell body,  swimming Vorticella is similar to a micro-scale submarine with the cilia being the propeller. Because other swimming micro-organisms of similar sizes are usually fully covered with cilia, Vorticella is a unique model organism for micro-swimmers.

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     My preliminary observation is that Vorticella shows a variety of swimming patterns depending on different levels of confinement. This can be seen in Table 1. They can swim upright if they have more room to swim around, or swim horizontally with limited space if they are more confined. When a Vorticella cell is upright, not much can be seen because one would be looking down at a cell as it is “standing”. In order to better image their movements and recognize them as cells, the cells must be confined so it is easier to pinpoint them when they are “lying down”, or in a 2D swimming state. Based on the preliminary observation, my hypothesis is that as the gap height of the 2D confinement decreases, and thus the volume allowed for Vorticella decreases, the swimming pattern of Vorticella will slow and become more two-dimensional.

     The main research activity of this project is to record and analyze swimming patterns of Vorticella in 2D confinements of various gap heights. First, Vorticella cells will be harvested onto petri dishes. When the bottom of the petri dish is scraped, their stalks are cut and the cells are detached from the bottom of the dish, granting them freedom to swim around. Water drops with cells in it will be placed on a slide glass, two spacers/shims will be placed on the two ends of the slide glass, and another slide glass will be put on top of the spacers, confining the cells. Thus, the cells in water will be sandwiched between two parallel slide glasses with thin plastic spacers, and the gap height of this confinement will be modulated by changing the thickness of the spacers between 50 μm and 500 μm. Afterwards, the sample will be placed on the stage of an inverted microscope, swimming of individual Vorticella cells will be captured with 4x magnification using a digital camera (24 frames per second).

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     Vorticella has been studied extensively in its sessile form due to its importance in ecosystems with engineering applications such as wastewater treatment. However, not a lot of observations have been done on the protozoan when it is in its swimming mode. This study is important because it is essential to furthering our limited knowledge on micro-swimmers and to inventing micro-scale robots swimming like Vorticella or submarines. In particular, studying the swimming pattern of Vorticella in 2D confinements is meaningful because such confinements represent engineering applications of micro-swimmers such as microfluidic channels. In addition, this study will advance understanding of the ecology of Vorticella and similar micro-organisms, by answering how these animals swim around near a solid surface in their natural habitats. On top of that, studying the swimming mode of Vorticella in depth can help engineering students studying fluid mechanics to better understand the content and application to real worlds.