| dc.description.abstract | Droplet evaporation is a fundamental process that is encountered in a wide range of scientific and technological fields, including microfluidics, surface coating, biomedical diagnostics, and water harvesting. Extensive research has revealed that the dynamic behavior of the contact line and contact angle during droplet evaporation plays a pivotal role in this phenomenon. Understanding and characterizing the dynamics of the contact angle and contact line are therefore of utmost importance in studying and optimizing droplet evaporation processes. However, the majority of previous studies have primarily focused on spherical droplets. It is important to note that during the evaporation process, non-spherical droplets can form due to alternating pinning and depinning phenomena. These non-spherical droplets exhibit distinct behaviors throughout the evaporation process, emphasizing the need for further investigation and characterization of this droplets.
The objective of this research is to explore the dynamic behavior of the contact line and contact angle during the evaporation of a droplet using digital holographic microscopy (DHM). DHM relies on the acquisition of a three-dimensional (3D) hologram of the droplet, covering its entire periphery, enabling a comprehensive analysis. Unlike conventional optical techniques, DHM enables the capture of intricate details, including the variations in contact angle at different points along the contact line and the rapid and irregular motion exhibited by the contact lines. To achieve this, two distinct droplet cases with different evaporation modes are carefully examined.
In the initial scenario, we examined the evaporation of an axisymmetric droplet, where the DHM enables us to capture the evolution of the contact line and contact angle. This case demonstrates a mode of evaporation characterized by a constant contact angle. By utilizing DHM, we were able to record the instantaneous local contact angles at various points along the droplet's periphery. The observations revealed minor variations in the local contact angles along the contact line throughout the entire evaporation process. Thus, as anticipated, the average value of all the local contact angles along the droplet's periphery remained constant for the majority of the evaporation duration.
The second case involves the investigation of a non-axisymmetric evaporating droplet, where the occurrence of alternating pinning and depinning events is observed. These phenomena lead to the formation of a non-spherical droplet, which in turn influences the uniformity of the instantaneous local contact angles along the droplet's contact line. In this scenario, a significant variation in the instantaneous local contact angles at the droplet's periphery is observed. The depinning and pinning events, leading to the rapid movement of the contact line and variations in the instantaneous local contact angles, occur within a brief time-span of milliseconds. These phenomena are effectively captured by DHM, thanks to its high temporal resolution, which surpasses that of conventional methods that lack the capability to observe such rapid dynamics.
The results of our study underscore the potential of DHM as an advanced technique that offers significant advantages over traditional optical methods in investigating the dynamics of contact lines and contact angles. | |