The contact line and evaporation feed back on each other
A sessile droplet typically evaporates in stages: first with a fixed base diameter and a shrinking contact angle, then with a fixed contact angle and a shrinking base, and finally with both changing [1]. Each stage is a different regime of contact-line motion, and each produces a different evaporation pattern. In binary mixtures the link is even more direct: as water evaporates preferentially from an ethylene glycol–water droplet, the mixture becomes richer in the less volatile component, and the droplet passes from a constant-radius mode into a depinning mode [3].
The feedback runs both ways. Surfactants and temperature change surface tension and the internal flows, and these changes are expressed at the triple line, the junction of liquid, solid, and air, which the authors describe as controlling the evaporation rate [1]. In colloidal droplets, adding surfactant can suppress the coffee-ring effect only above a concentration that depends on the particle load, and crossing that threshold depletes particles near the contact line without much changing the droplet's overall geometry or total evaporation time [5].
Pinning, depinning, and stick-slip are where the coupling becomes visible
The clearest signature of the coupling is the way the contact line sticks and then slips. In a simulated soil pore made of mixed hydrophobic and hydrophilic beads, a water droplet could be pinned on a Teflon bead while depinning on a glass bead, and the droplet switched between constant-radius and constant-area evaporation modes in a stick-slip pattern that the authors suggest minimizes surface energy [4]. Wettability also changed evaporation times: hydrophobic and mixed pores took about 39–40 minutes on average, versus about 34 minutes for hydrophilic pores [4].
On textured surfaces, molecular dynamics simulations have been used to characterize the pinning and depinning mechanisms themselves, which is one of the few ways to see the contact line at the scale where these events actually occur [2]. After depinning, the evaporation rate classically decreases; with surfactant present, an enhancement can appear late in the droplet's life, which the authors suggest could come from film evaporation near the triple line [1].
Why the coupling has been hard to pin down
The main obstacle is that the controlling variables are local and continuously changing. With surfactant, the local concentration near the triple line shifts as the droplet evaporates, making the evaporation rate complex to predict [1]. The forces that control depinning can be quantified, and a clear constant force has been found after depinning, but that equilibrium reflects friction, surface roughness, and the limit of surfactant concentration right at the line [1].
Measurement adds another layer of difficulty. Resolving contact-line motion and selective evaporation in a binary droplet required combining surface plasmon resonance for concentration with shadowgraph imaging for shape, and only then could the measured evaporation rates be checked against a diffusion model, which they matched to within about 6% at worst [3]. At the nanoscale, direct observation gives way to simulation [2]. These demands help explain why the coupling has received less attention than the evaporation rate alone.
Temperature can flip the direction of the surfactant effect
The coupling is not a fixed rule. At ambient temperature, around 20 °C, increasing surfactant concentration lowered the local evaporation rate, but at higher substrate temperature the same increase enhanced it [1]. Substrate heating on its own raises the evaporation rate [1]. So any statement about how surfactants and contact-line motion combine has to specify the temperature at which it was measured.
Sources used in this answer
Effect of temperature and surfactants on evaporation and contact line dynamics of sessile drops
Temperature and surfactants alter evaporation and contact-line dynamics in sessile droplets, with the triple line described as controlling the evaporation rate and with the surfactant effect on local evaporation reversing between ambient and higher temperature.
Pinning and depinning mechanism of the contact line during evaporation of nano-droplets sessile on textured surfaces
Molecular dynamics simulations characterize the pinning and depinning mechanisms of the contact line during evaporation of nano-droplets on chemically heterogeneous textured surfaces.
Selective evaporation and contact line motions of evaporating ethylene glycol–water mixture droplets
Ethylene glycol–water droplets evaporate in a depinning mode after a constant-radius mode, with water evaporating preferentially and measured evaporation rates matching a diffusion model to within about 6%.
Effects of heterogenous wettability on evaporation from a simulated soil pore: Stick-slip evaporative mode and contact line motion
In simulated soil pores, mixed wettability produces stick-slip evaporation and configuration-dependent evaporation times, with droplets sometimes pinned on hydrophobic beads while depinning on hydrophilic ones.
Effect of Particle Concentration on Surfactant-Induced Alteration of the Contact Line Deposition in Evaporating Sessile Droplets
Higher particle concentration lengthens pinning time and widens the ring, and the surfactant concentration needed to suppress the coffee-ring effect rises with particle concentration without significantly changing droplet geometry or total evaporation time.