Surfactant Function and Surface Tension: Lab Report
This lab report investigates the function and importance of surfactants through two household experiments. The first experiment examines surface tension by counting water drops held on a penny before overflow. The second uses milk, food coloring, and dishwashing liquid to observe how surfactants reduce surface tension. Results showed that 37 drops formed a dome on the penny before running off, and that adding dishwashing liquid increased the rate of food coloring movement from 1 mm/min to 3.5 mm/min. The findings are connected to pulmonary physiology, illustrating how surfactants coat alveoli walls, lower surface tension at the air-liquid interface, and prevent alveolar collapse during exhalation.
- Introduction: Background on surfactants and experiment objectives
- Materials and Methods: Step-by-step procedures for both experiments
- Results: Drop counts, distances, and rate calculations
- Discussion: Interpretation of results and respiratory system analogy
- Conclusion: Summary of surfactant importance and key findings
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What makes this paper effective
- The paper clearly maps each experimental element (milk, food coloring, dishwashing liquid) onto an anatomical analog (water interface, gas, surfactant), making the biological relevance concrete and accessible.
- Quantitative measurements — drop counts, distances in millimeters, and rates of movement — give the findings specificity and allow percentage change to be calculated, strengthening the empirical basis of the conclusions.
- A control condition (food coloring in milk without dishwashing liquid) is explicitly included and justified, demonstrating sound experimental design reasoning.
Key academic technique demonstrated
The paper effectively uses analogical reasoning to bridge a simple household experiment and a complex physiological process. By explicitly identifying each component of the experiment as a stand-in for a biological structure or substance, the author transforms an accessible observation into a meaningful model of pulmonary surfactant function — a technique common in introductory science writing.
Structure breakdown
The report follows a standard IMRaD (Introduction, Materials and Methods, Results, Discussion) structure with a brief Conclusion. The Introduction establishes background and hypotheses. Methods describe both experiments and the control in sequential steps. Results report raw data with calculations. The Discussion interprets findings in relation to the hypotheses and connects them to respiratory physiology, citing clinical consequences such as atelectasis. The Conclusion synthesizes the core takeaway concisely.
Introduction
The purpose of this lab report is to demonstrate the function and importance of surfactants using household products. Surfactants play an important role in the respiratory system, whereby they coat the alveoli walls of the lungs (National Library of Medicine [NLM], 2017). According to the NLM, the ability of surfactants to coat the alveoli walls is rooted in their surface-tension-reducing capabilities.
Two experiments were performed. The first experiment was designed to observe the surface tension created when drops of water are placed on a penny, and to determine why a penny can hold so many drops before the water runs off. The second experiment was performed to observe whether surfactants counteract the properties of water by lowering the surface tension of water molecules, and whether the absence of a surfactant could affect the respiratory system. This question is especially relevant because, as the NLM (2016) points out, the ability of surfactants to lower surface tension depends largely on the concentration of the surfactant on the surface of the alveoli. When surfactant concentration is low, it diminishes the pressure needed to inflate the alveoli during exhalation, leading to breathing problems (National Library of Medicine, 2017).
Materials and Methods
Two experiments were carried out using household products to demonstrate how surfactants work. The first experiment, which involved surface tension, was conducted before the effects of surfactant were explored. In this experiment, a penny was first washed, rinsed, and dried with a paper towel. A clean eyedropper was filled with tap water. The dry penny was placed on a flat surface, and individual drops of water were placed on its surface until the water ran off. The number of drops held before overflow was counted and recorded.
In the second experiment on the effects of surfactant, a clean shallow dish was placed on a flat surface and whole milk was poured in until it completely covered the bottom of the dish. A couple of drops of food coloring were then added to the milk using an eyedropper. The solution was observed for two minutes. After that time elapsed, the distance from the center of the food coloring to its outer edge was measured in millimeters using a ruler. The rate of movement per minute was then calculated to determine the rate in millimeters per minute.
Next, a couple of drops of dishwashing liquid were added drop-wise to the center of the dish. Two minutes later, the milk and food coloring were observed again, and the distance from the center of the food coloring to its outer edge was measured in millimeters. The rate of movement per minute was also calculated.
To control for the possibility of the dishwashing liquid reacting independently with the milk and food coloring, a control experiment was performed in which food coloring was added to the dish of milk without adding any dishwashing liquid.
Conclusion
The effect and role of surfactants depend on their ability to lower the surface tension of water, which is naturally high. These surfactants lower surface tension, ensuring that the gas and liquid interface interact. In the experiments described here, the dishwashing liquid acted as a surfactant, reducing the surface tension of the milk and enabling the milk and food coloring to mix. It is important to note, however, that because surfactants play a critical role in lowering surface tension within the respiratory system, their absence may be detrimental — leading to collapse of the alveoli during exhalation.
References
Han, S., & Mallampalli, R. K. (2015). The role of surfactant in lung disease and host defense against pulmonary infections. Annals of the American Thoracic Society, 12(5), 765–774.
National Library of Medicine. (2017). Surfactant dysfunction. https://medlineplus.gov/genetics/condition/surfactant-dysfunction/
Water Science School. (2019). Surface tension and water. Georgia State University. https://www.usgs.gov/special-topics/water-science-school/science/surface-tension-and-water
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