## 1 Introduction  
Electronic cigarettes (e-cigs) have been on the market for more than ten years. They have the same elements (Figure 1): an atomiser screwed or welded to a power unit. Atomisers are metallic cylinders with a coil unit that are sold empty. They are filled by a user with an e-liquid. In their design, the air inflow and aerosol outflow allow air circulation during the inhalation. The coil unit is also a metallic cylinder containing a resistive wire and a porous media draining the e-liquid from the tank (initial empty volume that is designed to receive and to stock the e-liquid) to the wire. In the beginning, they had the same design as classical cigarettes. They were mostly made as a closed unit. The power unit used to deliver a fixed power was sealed to the rest of the device. Then, power units began to be screw-off and rechargeable. Following this innovation, adjustable power units appeared in the market. The power regulation had paved the way for the development of a wide panel of atomisers and e-liquids. More than 33,000 e-liquids and 6000 electronic cigarettes and accessories have been declared between 2017 and 2020 to the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) [1]. E-cigs are commonly classified by the generation of products mostly based on designed characteristics. Many descriptions are available but there are not standardized characteristics that allow an objective classification. In a publication, authors reviewed the current classifications available on the web [2] describing each category of products (Figure 2). This classification mainly describes a story about the development of the devices since their apparition. Through this classification, there is not clear criteria allowing a strict classification of a device in a group. Despite this wide market, the electronic cigarette function has not changed. As the user activates the power unit, an electrical current is delivered to the resistive wire. In response, the wire heats and vaporizes the surrounding e-liquid. The generated aerosol is finally inhaled by a consumer. So, e-cigs are devices using a thermal and thermodynamic phenomenon to generate an aerosol and depending on the e-liquid composition to deliver an active molecule such as nicotine. The variety of vaping products available (e-cigs and e-liquids) offers a large panel of configurations for a consumer. The electronic cigarette functioning using standardized conditions was recently characterized [3]. According to the supplied power, three vaporization regimens were identified and described. At low powers, no aerosol is generated. This vaporization regimen is called under-heating. Increasing the power leads to the optimal regimen. In this one, the mass of vaporized e-liquid (MVE) according to the supplied power shows a linear trend. Then, above a power limit, this linearity is broken, and an over-heating regimen is reached. Consequently, the optimal regimen was characterized by two power limits: minimal and maximal powers. In a recent publication [4], authors link these regimens to the produced aldehydes and highlight a significant increase in their productions using powers higher than the maximal power. They also suggest a link between the boiling process and vaping. The maximal limit would correspond to the critical heat flux. Consequently, wire surface is a key parameter that defines the limits of the optimal vaping regimen. By extension, knowing its value allow the determination of these limits and a requirement of power range with a lower chemical risk. Additionally, we showed how vaping regimen (airflow rate) influenced the optimal vaporisation regimen [5]. The inhalation through an e-cig is commonly separated into two main behaviours (Figure 3): a mouth to lungs (MTL) or a direct lung (DL) inhalation. As illustrated, using MTL vaping regimen [6] reduces significantly the optimal vaporisation limits (lower minimal and maximal powers) and devices efficiencies.