Understanding the coffee ring effect: how it has led to advanced applications

The most cited article in the area of drying is on droplets. The article entitled “Capillary flow as the cause of ring stains from dried liquid drops,” has so far been cited more than 4860 times according to Web of Science. The article explains a fascinating and common phenomenon viz. why a drying droplet, such as a spilled drop of coffee, leaves a circular ring as its residue, rather than a uniform spot. It reveals how suspended solid particles flow, spread and dissipate during the liquid droplet drying process. The solid particles are uniformly distributed in an initial drop, but during drying they concentrate around the edges. Deegan et al. provided the first microscopic level description of this phenomenon, which is known as the “coffee ring effect.” They attributed the ring stains that develop as the droplets dry to capillary flow. According to Ball, the droplet’s lateral dimensions cannot simply decrease due to evaporation if the edge is fixed in situ. To replenish the liquid lost to evaporation while maintaining the contact line for continuity, there must be a net flow toward the edge. This flow carries suspended particles with it. The lack of a thorough theoretical model that can precisely forecast the drying kinetics and deposit structure is one of the main obstacles to understanding droplet drying. However, new developments in microscopy and imaging techniques have enabled researchers to directly examine the drying process and acquire an understanding of the underlying physics and chemistry behind the “coffee ring stains.” For instance, recent research has demonstrated that a droplet’s evaporation rate is influenced by a variety of variables, such as the droplet size, temperature and humidity of the surrounding environment and the characteristics of the surface it is deposited on. Using Controlled Evaporative Selfassembly (CESA) in a confined geometry provides a high degree of control over the drying dynamics and related flows, allowing for the creation of complex deposit patterns with a regularity that has never before been possible. These variables may have an impact on the deposit’s final structure, causing the development of a wide variety of geometric patterns such as coffee rings, spider webs or even larger structures that resemble volcanoes or arches or other geological formations. It is important to be able to manage the deposit structure and morphology since it can be exploited to develop unique structures in certain applications in electronics, optics and energy storage. The surface tension of liquid as well as humidity and temperature of the immediate environment, as well as the makeup of the surface on which the droplet is spilled can all have an impact on the shape, pattern and size of the stain. A stain may spread out more and appear less defined on a porous surface like paper or fabric or it may be more concentrated and have a more recognizable shape on a smooth, non-porous surface like glass or plastic. Due to the loosely packed pore structures between particles, ellipsoidal particles with less mobility in a suspension drop lengthen the time that air invades the stage of drying as compared to spherical particles, thereby reducing the effect of coffee ring stain. In biology and biotechnology, droplet drying has been used to explore the behavior of cells and proteins. In biomedical applications, the concept of exploiting patterns produced by the drying of biological fluids to diagnose disorders is interesting. By analyzing, comprehending and interpreting patterns formed by evaporating droplets, a simple and rapids means for screening a range of life-threatening illnesses may be possible. Researchers have indeed already achieved important strides on this. The coffee ring effect has also been employed in the electronics sector to deposit nanoparticles in a predetermined pattern to create conductive coatings for printed electronics. Using the coffee ring effect, anti-reflective coatings with high transmission and low reflection have been developed in the field of optics. Compared to conventional coatings, anti-reflective coatings made via the coffee ring effect have some benefits; they are inexpensive, simple to make and work with a variety of materials, such as metals, plastics and glassware. We believe readers interested in drying science and technology will reflect on the complexity and nature’s creativity next time they take a careful look at coffee stains they have inadvertently generated. Maybe they will reimagine innovative drying applications in various industrial sectors this way!