The optically controlled assembly of suspended particles from evaporating sessile droplets is an emerging method to realize on-demand patterning of particles over solid substrates. Though dynamic control over the assembly of microparticles is possible, limited success has been achieved in nanoparticle patterning, especially in the case of metallic nanoparticles. This work demonstrates a simple light-directed patterning of Gold nanoparticles (Au NPs) based on the thermoplasmonically-controlled liquid flow. 
Method: The on-demand assembly of Au NPs was realized via thermoplasmonically- controlled liquid flow inside an evaporating sessile drop (3 μL, 0.8 nM Au NP droplet). A diode laser (532 nm, Power - 80 mW) focused into a circular spot radius of 75 µm was utilized to induce the localized heating. We performed optical microscopy and 3D profilometry to characterize the final particle deposit. 

Results: The plasmonic excitation led to a temperature difference of about 18 °C between the irradiated region and the three-phase contact line of the pinned droplet, resulting in the particle assembly at the irradiation zone in response to the thermocapillary flow created inside the droplet. Particle streak velocimetry experiments and analysis confirm the existence of a strong thermocapillary flow which counteracts the naturally occurring evaporative convection flows (up to t/tf ~0.8, where tf denotes the total evaporation time). 3D profilometry analysis revealed a 102 times enhancement in the deposit height upon irradiation, in comparison to the size of individual NPs (~44 nm). To test the versatility of the developed method, we tried different illumination patterns using photomasks/sequential scanning, which led to various patterns, including off-center, two-spot, and line deposit.

Conclusions: We demonstrate an efficient optical strategy for realizing patterns of Au nanoparticles based on thermoplasmonically-controlled liquid flow. A variety of patterns such as central deposit, off-center deposit, multi-spot deposit, and lines could be inscribed by simply varying the illumination pattern.

 

Affiliated Person
Farzeena C

Publications
[1]    C. Farzeena and S. N. Varanakkottu, ‘Patterning of Metallic Nanoparticles over Solid Surfaces from Sessile Droplets by Thermoplasmonically Controlled Liquid Flow’, Langmuir, vol. 38, no. 6, pp. 2003–2013, Feb.2022.



1