Study On The Deposition Morphology Of Particle Suspension Droplets Caused By Evaporation

Coffee-Ring is a common natural phenomenon in life.Many scholars have conducted in-depth research on this phenomenon from theories,simulations and experiments,and its mechanism has become increasingly clear and has potential applications in many fields.When a colloidal droplet evaporates on the horizontal wall,the internal flow carries tiny particles close to the contact line,and the annular deposition structure formed after the evaporation is called Coffee-Ring.The self-assembly behavior of particles during the evaporation of colloidal droplets has important applications in many fields.In biomedicine,for example,Coffee-Ring phenomenon can be used to diagnose diseases,sort large biological particles,concentrate samples and enhance analytical signals.In microelectronics,it can be used to prepare microcircuits and transparent electrical circuits.The suppression of the Coffee-Ring effect can be applied in the field of spraying to achieve uniform distribution of the coating.Some scholars manipulate the particles through external field,such as sound field,magnetic field and electric field,to achecve accurate deposites.The assembly of the particles by droplet evaporation does not require additional energy,and its simple,easy-to-operate features attract more and more people to study this phenomenon.In this paper,through theoretical model analysis combined with Matlab simulation,the change of height,contact angle and internal flow velocity during the evaporation of a droplet with constant contact area is obtained.It is found that the height and contact angle of the droplet decrease linearly with time,the flow velocity inside the droplet increases exponentially from the center to the edge,and the flow velocity increases exponentially with time at any position.In the region close to the air-liquid interface,the vertical component velocity is much greater than the radial component velocity because of the constraint of the spherical cap structure.Considering that the temperature change caused by droplet evaporation,the Marangoni flow occurrs and gradually weakens with evaporating time,and disappears before the end of evaporation.All else being equal,considering the change of the Ma number because of the additive,the larger the Ma number,the more intense the Marangoni flow.The forces of suspended particles in droplet is analyzed,which is mainly affected by flow resistance.The evaporation experiment was carried out with suspension of micron-sized polystyrene particles.The movement of the particles,the behavior of the particles at the contact line and the deposition pattern after evaporation were observed using an inverted microscope.The smaller the diameter of the particle,the more obvious the Coffee-Ring effect,and the particle mass concentration is too high or too low to form a ring deposition.Particles congregated at the contact line will retreat toward the center and the reverse coffee ring phenomenon appears: the contact angle decreases with time,the gas-liquid interface squeezes the particles toward the center of the droplet,and the particles with small size are distributed close to the contact line,while those with large size are far away from the contact line,forming particle sepearation based on particle size.The particles do not satisfy the wedge structure throughout the evaporation process.When the contact angle is reduced to a certain extent,the downward pressure force of the gas-liquid interface to the particle is too large,causing the gas-liquid interface to deform and the wedge-shaped structure to be destroyed,and the particle size is larger,the earlier the wedge structure is destroyed.The particle velocity near the contact line increases exponentially with time,which is consistent with the simulation results.The effects of surfactant(sodium dodecyl sulfate SDS)and pH on particle deposition pattern were explored.The addition of the SDS solution in droplet causes the aggregated particle ring at the contact line to spread wider and the ring’s width increases as the SDS concentration increases.Different pHs change the surface charge of the particles and affect the interaction between the gas-liquid interface and the particles,which in turn changes the deposition morphology.The study of microfluidic motion in the evaporation process of particle suspensiond involves multi-disciplinary knowledge and its potential application in many fields is worth exploring.In this paper,the Coffee-Ring phenomenon was systematically analyzed through theoretical analysis,numerical simulation and experimental verification.The phenomenon that adding SDS makes the Coffee-Ring wider has not been found in previous studies.It is expected to provide reference for the complex sedimentary morphology control of colloidal droplet and practical applications in the future.