Killing microbes with Bubbles

August 11, 2020 by anon


If you look into Google search statistics for the search ‘wash hands with soap’, you will find that the term saw a tremendous growth in popularity in the month of March.[1] This trend is quite easily explained as this was around the time when people started to understand the importance of hand washing with soap as a preventive measure towards the spread of the infamous COVID-19 pandemic. Almost every health advisory had the same statement, on their preventive-measures checklist.

A basic understanding of high school chemistry allows us to explain the cleaning mechanism of soaps. To put it simply, soaps develop micelles in solution. Micelles are supramolecular assemblies of molecules having both hydrophobic (water-repelling) and hydrophilic (water-loving) ends. The protective cell membrane of almost all microorganisms is made of a lipid membrane which also has a double layered micelle like structure. When we wash hands with soap, in an attempt to evade water, the hydrophobic ends of soap micelles, enter the lipid membranes of microorganisms. From a microorganism’s point of view, this means the rupture of their protective cell membrane and eventual death.

This cleaning process of soap is often completely attributed to micelle formation, but what about the bubbles? Most articles available on the internet suggest they have no function whatsoever, and are included in the cleaning product to satisfy the psychological need of users to see bubbles as an indicator of something getting cleaned. In fact, many soap manufacturers add chemicals specifically to create bubbles and fulfill this marketing strategy. However, the science of cavitation bubbles suggests the opposite.

The term, cavitation is defined as the formation of small vapor bubbles inside an initially homogeneous liquid solution. A bubble forms when a sudden decrease in pressure occurs inside the liquid medium or the temperature is increased. Eventually, the bubble collapses due to the external pressure of the surrounding liquid and when it does, it is nothing short of a violent implosion. The previous statement may easily seem like an over exaggeration, but locally, near the immediate vicinity of the bubble’s environment, calling it ‘violent’ may be an understatement. According to studies, a bubble collapse can result in pressure shock waves up to 100 megapascals (1000 bar)[2]. To give you an idea of how large this value is, the pressure caused by a lethal shock wave from a bomb explosion typically is in the range 414–552 kilopascals (4.14 – 5.52 bar)[3]. In addition to the intense pressure during the bubble collapse, extreme temperature ‘hot-spots’ are generated at the center of the bubble with temperatures in order of 1000 K.[4] Although, these temperatures and pressures only exist for a fraction of a microsecond and the intensity of shock waves rapidly decreases as it moves away from the center of the bubble; from the perspective of a microorganism, a bubble collapse can be devastating. All the mechanical phenomena which occur with a bubble collapse have been reported to damage the protective membrane layer of different microorganisms.[5],[6]. The high temperature hot spots formed further aid these processes by affecting structural integrity of the protective layer.[7] The implosion of bubbles and the formation of these hotspots can lead to generation of free radicals such as the hydroxyl radical from the homolytic cleavage of water. The hydroxyl radical is a strong oxidant which can readily form H2O2, a known biocide. This results in the oxidative degradation of the protective lipid bilayer of microorganism and causes additional damage to the microorganism’s internal components.

The implosive nature of the bubble collapse, actually pulls nearby particles towards the bubble center. This is a promising aspect when it comes to removing microorganisms stuck to a surface.[8] This was nicely visualized in a study conducted by Jung et. al.[9] where effects of bubble collapse on a nearby spherical particle was observed. The short clip below shows that the particle moves towards the region of bubble collapse.
(Video taken from https://www.youtube.com/watch?v=mceZ3Ep20AQ )

According to the paper, the method has the potential to remove microbes from surfaces of agricultural produce without the use of chemical additives. Cavitation bubble treatments were found to be as effective as chlorine in studies conducted by Lee et. al. [10] for removing microbes from surfaces of tomatoes.
However, research focused on cavitation-bubble assisted inactivation of microorganisms has only recently gained attention. The underlying mechanisms of bubble-microorganism interactions is an aspect which needs to be elucidated further. That being said, the idea of literally bombing microorganisms with soap bubbles remains extremely fascinating and intriguing.

References :

[1] https://trends.google.com/trends/explore?q=wash%20hands%20with%20soap
[2] Brennen, C.E., 2014. Cavitation and bubble dynamics. Cambridge University Press.
[3] Committee on Gulf War and Health: Long-Term Effects of Blast Exposures; Board on the Health of Select Populations; Institute of Medicine. Gulf War and Health, Volume [9] Long-Term Effects of Blast Exposures. Washington (DC): National Academies Press (US); 2014 Apr 14. 3, Pathophysiology of Blast Injury and Overview of Experimental Data. Available from: https://www.ncbi.nlm.nih.gov/books/NBK202251/
[4] Extreme conditions during multibubble cavitation: Sonoluminescence as a spectroscopic probe (https://doi.org/10.1016/j.ultsonch.2010.12.012 )
[5] Alternative methods of microorganism disruption for agricultural applications (https://doi.org/10.1016/j.apenergy.2013.08.085)
[6] Inactivation of microbes using ultrasound: a review (https://doi.org/10.1016/S0168-1605(03)00075-8)
[7] Inactivation of microbes using ultrasound: a review (https://doi.org/10.1016/S0168-1605(03)00075-8)
[8] Cavitation Bubbles Remove and Inactivate Listeria and Salmonella on the Surface of Fresh Roma Tomatoes and Cantaloupes (https://doi.org/10.3389/fsufs.2018.00061)
[9] Particle Motion Induced by Bubble Cavitation (https://doi.org/10.1103/PhysRevLett.114.214501)
[10] Cavitation Bubbles Remove and Inactivate Listeria and Salmonella on the Surface of Fresh Roma Tomatoes and Cantaloupes (https://doi.org/10.3389/fsufs.2018.00061)


Guest post written by Davis Thomas Daniel

After his bachelors in Chemistry from St. Stephen’s College, Delhi, Davis moved to Germany in 2017 to complete his Masters in Chemistry specializing in Magnetic Resonance from University of Bonn. He is currently pursuing his doctoral degree at Forschungzentrum Jülich in Germany where he investigates redox processes in Organic Polymer Batteries using Electron paramagnetic resonance spectroscopy.
He enjoys cooking, surreal humor, casual programming, visiting museums and writing poetry.

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