Session F17: Drops: Dynamic Surface Interactions

Contact line friction in aspirated sessile drops

We study the macroscopic properties of sessile drops deposited on and aspirated from silanized glass slides. By observing the contact angle and drop width simultaneously, we can determine the depinning angle and motion of the contact line as the drop is removed. Young’s formulation and models of contact angle hysteresis both predict a constant contact angle after depinning. In contrast, we observe that the contact angle continues to change during the drop removal process. Our data suggest a contact line friction that opposes the motion of the drop’s edge. As has been seen previously, this force is dependent on the contact line speed [1]. We show that it depends also on the concentration of the solute and the amount of time that the drop has been in contact with the surface.

[1] Gao, N., Geyer, F., Pilat, D. W., Wooh, S., Vollmer, D., Butt, H. J., & Berger, R., Nat. Phys. 14(2), 191-196 (2018).   

Droplets on a wall: moving contact lines with the Immersed Boundary Method

In this work, we propose and test four techniques for simulating the 2D Immersed Boundary (IB) method for moving droplets on a wall. The first technique defines a moving contact line (MCL) model and implements Navier slip at an immersed solid boundary. The static and dynamic contact line angle are endogenous instead of prescribed. The second technique simulates both a surface tension force and an unbalanced Young's force with one general equation that does not involve estimating local curvature. The third technique splices the liquid-gas interfaces to handle the coalescence and separation of liquid droplets or gas bubbles. The forth technique re-samples the liquid-gas interface markers to ensure a near-uniform distribution without exerting artificial forces. Then, we test our techniques using some benchmark cases, including a slipping droplet on a wall and a rising bubble, some convergence tests, and some other qualitative tests.   

Dynamic stabilization of toroidal drops in linear flow

Viscous drops, subject to a linear flow in an immiscible viscous fluid, deform and, under certain conditions, attain stationary shapes. When the outer flow is a combination of rotation and axisymmetric extensional or bi-extensional flow, yielded shapes are flat drops, flat drops with dimples, and toroidal drops. The toroidal drops in compressional flow are highly unstable. Under the combined action of rotation and extensional flow, two toroidal stationary shapes exist at the same conditions.  The torus with higher major radius is stable with respect to the axisymmetric disturbances, while the second one is unstable. Any perturbation of an unstable shape results in either collapse of the torus to a simply connected shape, or to torus extension. 

We applied control methods to stabilize toroidal drops in the linear flow. As a control variable we chose the intensity of axisymmetric (non-rotational) component of the outer flow. The control was designed using a simplified two-state dynamic model and tested on non-simplified dynamic equations for drop deformation. We succeeded to stabilize all the toroidal shapes obtained previously. Moreover, such a dynamic control enabled a construction of new shapes - almost collapsed highly unstable tori - that could not be previously obtained.   

Drop oscillation dynamics on viscous thin immiscible liquid films: slip to pin transitions

Drop oscillation dynamics as well as drop mobility studies on thin liquid films such as on lubricant infused surfaces (LIS) have been developed extensively in recent years - relevant for many applications such as in water harvesting and enhancing condensation. However, the influence of how drop oscillation behaves in tandem with the viscous resistance on LISs remains unclear. In this study, the droplet height, contact angle and frequency were observed by a high-speed camera from a lightly deposited water drop onto liquid oil films of various viscosities and types. The droplet contact line mobility was used to categorize the natural oscillation frequencies to either the slip contact line (SCL) or pinned contact line (PCL) cases. By comparing the experimental frequency results with SCL and PCL numerical simulations, we find good agreement between the two methods and demonstrate a transition of droplet oscillation from the SCL to the PCL cases while varying the viscosity and wettability of the liquid film at fixed film thicknesses.   

Effect of surfactants on drop formation in microfluidic channels

Emulsions are widely used in applications including inkjet printing, healthcare, pharmaceuticals [1,2]. Surfactants are commonly added during drop formation to modify the interfacial properties and improve the stability of emulsions. The presence of surfactants affects the drop formation process and the final size.  In recent studies [3,4,5] it was found that high dispersed phase flowrates enhance convection and surfactant mass transfer to the interface, which justifies the reduced interfacial tension values at short formation times.

 

In order to investigate velocity fields and possible circulation patterns during the fast drop formation process, high-speed μPIV was used. Drops were generated in a flow-focusing glass microchannel. Previously it was found that absorption and mass transfer rates compete with drop formation times [3], thus it is important to study mixing inside the drops. Results revealed that convection is more prominent in the surfactant-free cases and a decrease in circulation patterns is seen with increasing surfactant concentration, which is in agreement with previous works [6]. Numerical simulations were also performed of the drop formation in the microfluidic channel and excellent agreement was found for drop size, drop formation time and circulation patterns.   

Role of interfacial rheology on the dynamics of confined droplets.

The development of droplet-based microfluidics addresses the question of predicting the velocity of these droplets when they are pushed by a carrier phase. In this work we focus on non-wetting droplets, i.e. there is a lubrication film between the droplet interface and the channel wall. These droplets are squeezed between the channel walls, i.e. they adopt a pancake-like shape. It is well known that the dynamics of droplets depend strongly on the physicochemical properties of the solutions, in particular on the solubility of the surfactant that is used.  Typically, an accumulation of surfactant at a stagnation point of the interface can generate Marangoni stresses, leading to an increased dissipation in the lubrication film, and thus to a decrease in droplet velocity. We report experimental studies on the dynamics of droplets in a microfluidic Hele-Shaw cavity, by investigating the impact of the solubility of the used surfactants on their velocity. By doing so, we highlight that the droplet velocity dependence on their radius is reversed as soon as the ionic surfactants used have more than 12 carbons on their hydrophobic tail.

In order to investigate the local signature of the surfactant along the lubrication film, we have set up a Reflection Interference Contrast Microscopy (RICM) device with which the thickness of the lubrication films can be measured. Able to reconstruct the topography of the droplet/external phase interface, the local properties of the interface can be extracted.