Citation: Lee, J.; Murad, S.; Nikolov, A. Ballpoint/Rollerball Pens: Writing Performance and Evaluation. Colloids Interfaces 2023, 7, 29. https:// doi.org/10.3390/colloids7020029 Academic Editors: Marzieh Lotfi, Reinhard Miller and Mohammad Firoozzadeh Received: 6 March 2023 Revised: 29 March 2023 Accepted: 31 March 2023 Published: 4 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). colloids and interfaces Article Ballpoint/Rollerball Pens: Writing Performance and Evaluation Jongju Lee , Sohail Murad and Alex Nikolov * Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA * Correspondence: nikolov@iit.edu; Tel.: +1-312-567-5980 Abstract: Here, a brief history of the development of the ballpoint/rollerball pen and the fountain pen is presented. Their principle of operation is analogous that of multipart microfluidics-type devices, where capillarity–gravity drives the ink, a complex fluid, to flow in the confinement of a micrometer-sized canal or to lubricate a ball rotating in a socket. The differences in the opera- tional writing principles of the fountain pen versus the ballpoint/rollerball pen are discussed. The ballpoint/rollerball pen’s manner of writing was monitored using lens end fiber optics and was digitally recorded. The ball rotation rate per unit length was monitored using a piezoelectric disk oscilloscope technique. The role of ink (a complex fluid) chemistry in the wetting phenomenon is elucidated. We also discuss methods for studying and evaluating ink–film–ball–paper surface wetting. The goal of the proposed research is to optimize and improve the writing performance of the ballpoint/rollerball pen. Keywords: ballpoint pen; fountain pen; microfluidics; wetting; piezoelectric disk transducer; ink-complex fluid; modeling 1. Introduction “Verba volant, scripta manent” (Calus Titus). The development of writing changed the world and began a new era of civilization. Humans used spoken languages for a hundred thousand years before they were inspired to mark their ideas down and preserve knowledge for future civilizations. Handwriting devices, such as those used with clay tablets (tokens) and papyrus, were used to write symbols expressing verbal meaning [1]. These early devices included tools with a sharp triangular tip, reed-like pens, and pens made from a goose or swan feather. Before the cur- rent fountain pen design appeared, many people contributed to its development, including Aristotle (384–332 BC) and Leonardo da Vinci (1452–1519) [2]. Fountain pen lovers are usually hobby enthusiasts who enjoy pen writing and calligraphy. The price of a fountain pen with good writing performance is more than USD 100, much more expensive than the popular USD 1 ballpoint/rollerball pen. To evaluate the cost of the writing performance of fountain pens and ballpoint pens, the design and operating principles were investigated. Figure 1 depicts the main features of fountain pens and ballpoint pens. The two handwriting devices’ operations are analogous to multipart microfluidics- type devices in which capillarity–gravity drives the ink, a complex fluid, to flow in the confinement of a micrometer-sized canal or to lubricate a ball rotating in a socket. They have a common feature: the delivery of ink through hydrostatic pressure. However, the ink delivery mechanisms for writing are very different: a nib with a slit versus a ball in a socket. The fountain pen’s ink–paper writing, i.e., the ink–cartridge–feeder–nib–slit and the air bubble capillary valve, were described by us in a recently published paper [3]. The fountain pen ink feeder (the unit between the ink cartridge and nib) has two canals: an ink canal and an air canal (see Figure 1A). During writing, the ink flows from the cartridge through the ink feeder canal to the nib slit. Once the holding air pressure builds up in the cartridge, the ink flow is reduced and stopped. To continue the ink flow while writing, the air holding pressure in the ink cartridge needs to be reduced. The air bubble has to move through Colloids Interfaces 2023, 7, 29. https://doi.org/10.3390/colloids7020029 https://www.mdpi.com/journal/colloids