Growth of solid conical structures during multistage drying of sessile poly(ethylene oxide) droplets (1003.5150v1)

Abstract: Sessile droplets of aqueous poly(ethylene oxide) solution, with average molecular weight of 100 kDa, are monitored during evaporative drying at ambient conditions over a range of initial concentrations $c_0$. For all droplets with $c_0 \geq 3%$, central conical structures, which can be hollow and nearly 50% taller than the initial droplet, are formed during a growth stage. Although the formation of superficially similar structures has been explained for glass-forming polymers using a skin-buckling model which predicts the droplet to have constant surface area during the growth stage (L. Pauchard and C. Allain, Europhys. Lett., 2003, 62, 897-903), we demonstrate that this model is not applicable here as the surface area is shown to increase during growth for all $c_0$. We interpret our experimental data using a proposed drying and deposition process comprising the four stages: pinned drying; receding contact line; bootstrap growth, during which the liquid droplet is lifted upon freshly-precipitated solid; and late drying. Additional predictions of our model, including a criterion for predicting whether a conical structure will form, compare favourably with observations. We discuss how the specific chemical and physical properties of PEO, in particular its amphiphilic nature, its tendency to form crystalline spherulites rather than an amorphous glass at high concentrations and its anomalous surface tension values for MW = 100 kDa may be critical to the observed drying process.

• The paper proposes a novel four-stage drying mechanism for sessile poly(ethylene oxide) droplets, leading to central conical structures above specific concentrations, challenging existing skin-buckling models.
• Experimental results show conical structures form for PEO concentrations exceeding 3%, growing up to 50% taller than the initial droplet, with critical concentration thresholds identified around 49% and 73%.
• These insights improve the understanding of polymer drying dynamics and pattern formation, with practical implications for industries using PEO in coatings and food technology and potential for controlled drying processes.

Overview of the Growth Dynamics of Solid Conical Structures During Drying of Poly(ethylene oxide) Droplets

The paper investigates the intriguing phenomenon of solid conical structure formation during the drying of sessile droplets of aqueous poly(ethylene oxide) (PEO) solutions. The authors demonstrate that these structures emerge through a novel drying mechanism, distinctly diverging from established models, and provide experimental and theoretical insights into the stages of this drying process.

Experimental Insight

The authors conduct an in-depth experimental paper on PEO droplets, with an average molecular weight of 100 kDa, subject to the common drying conditions. By varying initial concentrations (c₀) from 1% to 45%, the paper captures the conditions under which conical structures form. Notably, for concentrations exceeding 3%, the drying process results in central conical structures exceeding the initial droplet height by up to 50%. The authors utilize digital imaging and computational methods to extract and analyze critical droplet parameters such as droplet height, volume, and surface area during the drying evolution.

Novel Four-Stage Drying Mechanism

The authors propose an alternative model comprising four distinct drying stages:

1. Pinned Drying: The contact line remains fixed, maintaining a constant droplet base radius while height and contact angle decrease.
2. Receding Contact Line: Driven by dewetting transitions, the droplet's contact line retracts rapidly.
3. Bootstrap Growth: Solid spherulites precipitate at the contact line, lifting the liquid phase, and forming a conical structure.
4. Late Drying: Water within the spherulites evaporates, occasionally resulting in liquid ejection from the structure.

This model diverges from the skin-buckling mechanism observed in glass-forming polymers like dextran by demonstrating an increase in surface area during the droplet growth phase.

Results and Discussion

The formation of the structures is closely tied to the unique properties of PEO, particularly its amphiphilic nature and tendency to form crystalline spherulites at high concentrations. The data reveal growth dynamics that refute the applicability of glassy skin formation and instead illustrate a scenario where surface magic maintains integral to growth.

Experimental data reveal critical thresholds: a minimum droplet height coinciding with saturation concentration at an average of 49±8% aligns well with known PEO saturation concentrations. The growth continues until further crystallization occurs as the droplet approaches a concentration of 73±6%. The paper provides substantial qualitative and quantitative tools to predict the occurrence of solid conical structures based on initial droplet concentration and contact angles.

Conclusions and Future Implications

The novel drying mechanism presented in this paper elucidates the complexity of drying in polymer solutions and highlights the important interplay between polymer concentration, molecular configuration, and evaporation dynamics. With practical implications for industries utilizing PEO in coatings and food technology, these insights contribute significantly to the theoretical understanding of polymer drying and pattern formation.

The researchers suggest further exploration into the mechanistic roles of PEO's surface characteristics and interactions with substrates, which could lead to controlled drying processes for specific application needs. This paper opens avenues for further research into the dynamics of polymer deposition processes, with potential applications across various technological fields.