Generation Of Levitated Droplets Above Large Curvature Air-Liquid Interface Induced By Localized Photothermal Effect And Its Dynamic Characteristics

Droplet microfluidics can be widely applied to biochemical analysis,medicine,self-assembly of micro/nano materials,food quality monitoring,environmental pollutant monitoring,etc.The manipulation of small droplets is the key to realize the above-mentioned applications.Among them,droplet levitation has received extensive attention and wide exploitation due to negligible friction resistance and effective avoidance of cross-contamination,etc.Presently,droplet levitation can be achieved by magnetic levitation,acoustic levitation,electrostatic levitation and photothermal levitation.Among them,photothermal levitation realizes the droplet levitation via the balance between the photothermal effect induced air-lift force and droplet gravity,which has no special requirement for droplets.Moreover,the photothermal levitation offers the advantages of fast response and good flexibility and so on,making it become one of important research highlights in the field of photothermal effect based droplet manipulation.Previous studies mainly used liquid water to generate levitated droplets.It has been demonstrated that levitated droplets cannot be directly generated above large-curvature interface and there exists a small-curvature interface limitation encountered in liquid water.Moreover,how the interface morphology and liquid properties affect the generation of levitated droplets remains unclear.There is still a lack of a unifired criterion to judge whether the levitated droplets can be generated by this light strategy or not.The dynamic behaviors of generating levitated droplets also remain unclear.To address these issues,this thesis is directed to the study on the generation of levitated droplets above the interface of liquids with extremely small saturated vapor pressure and the dynamics.The main outcomes of this thesis are summarized as below.（1）It is found that the levitated droplets can be directly generated above the interface of aqueous ethylene glycol with extremely small saturated vapor pressure and large-curvature.Even with critically high curvature in response to the contact of 141^o,the levitated droplet can still be directly generated,which breaks the limitation of small curvature encountered in liquid water.Via comparison between aqueous ethylene glycol and water,it is revealed that although aqueous ethylene glycol has low evaporation rate,condensed droplets can still be formed because of its extremely small saturated vapor pressure.Low evaporation rate can form stable creep flow near the laser heating region,which benefits the droplet levitation other than blowing away.Therefore,extremely small saturated vapor pressure plays an important role in generating the levitated droplets above large-curvature interface.The effects of the laser power and interface morphology on the generation of the levitated droplets have been revealed.It is shown that there exist three stated for condensed droplet,i.e.,no droplet levitation,stable levitation and unstable levitation,above both small and large curvatures.The size of stably levitated droplets increases with increasing the laser power.The diameter of the levitated droplets increases from～16μm at 50 m W to～32μm at 100 m W.（2）It is found that not only ethylene glycol,but also other liquids with extremely small saturated vapor pressure,such as 1,2-propanediol and 1-2-butanediol,can allow for direct generation of levitated droplets above large-curvature interface,which further confirms the importance of the extremely small saturated vapor pressure in the formation of levitated droplets.A unified model is established to describe the generation of photothermal levitation of droplets.The nucleation process of supersaturated vapor is explored,in which the nucleation energy barrier is employed to judge the nucleation followed by the formation of condensed droplets.The analytical results are in good agreement with experimental data.Besides,the strength of the air flow induced by the photothermal effect is characterized by Grashof number and the relationship between the gravity of condensed droplets and supersaturation is achieved by using classical model of droplet growth.Based on this,a unified criterion,i.e.dimensionless Levitation number is proposed to describe the competition between the air lift and gravity of condensed droplets,which can successfully predict the status of the droplet levitation,including no droplet levitation,stable levitation and unstable levitation.It is found that stably levitated droplets can be directly formed when Levitation number falls in the range of 2.25×10^-4～6×10^-3.（3）The gas flow behaviors above the droplets with extremely small saturated vapor pressure are investigated.It is found that the air lift flow above the air-liquid interface mainly relies on the thermal convection because of extremely small saturated vapor pressure and high boiling point.They all exhibit stale creeping flow.The variations in the location,vertical velocity and size of condensed droplets near the laser beam are achieved.It is found that because of the interaction between the growth of condensed droplets and air flow,the condensed droplets experience a damped oscillating motion process before stable levitation.The Maxey-Riley equation is employed to describe the damped oscillating motion process of condensed droplets,with which the underlying mechanism of the decreased vertical velocity of condensed droplets in relation to the supersaturation is revealed.At low laser power,the condensed droplets grow slowly and it takes a long time for the damped oscillating motion of condensed droplets before stable levitation.While for large laser power,the supersaturation increases,maximum vertical velocity of condensed droplets decreases more quickly,and the time for the damped oscillating motion becomes shorter.As a result,condensed droplet can more rapidly evolve to levitated droplets.