Droplet microresonators for probing soft and biological systems

Pirnat, Gregor (author), Humar, Matjaž (mentor)

Droplet microresonators can support whispering gallery mode (WGM) optical resonances, which are highly dependent on the size and shape of the droplet. Even small perturbations in these two parameters cause distinct changes in the droplet's WGM spectra, providing a precise optomechanical sensing platform. Relying on the spectroscopy of WGMs we developed WGM tensiometry, a method for measuring interfacial tension (IFT) of the interface between the droplet microresonator and the surrounding liquid, and WGM elastometry, a method for measuring local stress and Young's modulus of a soft matrix surrounding the droplet. WGM tensiometry involves generating a dye-doped microdroplet of an optically clear liquid attached to the end of a glass microcapillary that can support WGMs while submerged in an immiscible continuous liquid phase. The size changes of the droplet are monitored via WGMs while applying finely tunable pressure through the microcapillary. The IFT is determined from the droplet size and the equilibrium pressure. This method allows the use of droplets as small as 8μm, requiring minimal sample volumes. Compared to other tensiometry methods, WGM tensiometry does not require the densities of the two liquids to be known in advance. Additionally, IFT measurements can be performed in non-equilibrium states when the droplet size or the chemical composition of the continuous phase changes over time. WGM elastometry is based on the elastocapillary interaction between the droplets and the surrounding soft matrix. We used gelatin hydrogel and mouse tissues as examples. The matrix was controllably deformed and the deformation response of the droplets was measured via modification of the WGM spectrum to determine the local anisotropic stresses, which could be as small as a few pN/μm^2. Additionally, Young's modulus of the matrix in the range 1kPa to 35 kPa, which covers most human soft tissues, could be measured. The developed methods enable precise determination of the mechanical properties of fluids and soft materials, relevant to a wide range of applications, including the study of biological processes.

• Language: English
• Keywords: microdroplets, whispering gallery modes, interfacial tension, liquid inclusions, elastocapillarity, soft materials, Young's modulus
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-162662
• COBISS.SI-ID: 209650947
• Publication date in RUL: 26.09.2024

Secondary language

• Language: Slovenian
• Title: Kapljični mikroresonatorji za preučevanje mehkih in bioloških sistemov

Abstract

Mikroresonatorji na osnovi kapljic lahko podpirajo ``whispering gallery'' (WGM) optične resonance, ki so močno odvisne od velikosti in oblike kapljice. Tudi najmanjše spremembe v teh dveh parametrih povzročijo znatne spremembe v WGM spektru kapljice, kar je osnova za natančno optomehansko senzoriko. Z uporabo WGM spektroskopije smo razvili metodo imenovano WGM tenziometrija, za merjenje površinske napetosti na stiku med kapljičnim mikroresonatorjem in okoliško tekočino, ter metodo, imenovana WGM elastometrija, za merjenje lokalne napetosti in Youngovega modula mehke matrike, ki obdaja kapljico. WGM tenziometrija vključuje generiranje in kontroliranje velikosti fluorescentno obarvane mikrokapljice narejene iz optično čiste tekočine, ki je pritrjena na konico steklene mikrokapilare in potopljena v okoliško tekočino. Mikrokapljice lahko podpirajo WGM načine, s katerimi spremljamo spreminjanje velikosti kapljice, medtem ko skozi mikrokapilaro apliciramo natančno nastavljiv tlak. Površinsko napetost se določi na podlagi velikosti kapljice in ravnovesnega tlaka. Ta metoda omogoča uporabo kapljic velikih le 8μm in zahteva minimalne količine vzorca. V primerjavi z drugimi metodami tenziometrije pri WGM tenziometriji ni potrebno vnaprej poznati gostote obeh tekočin. Poleg tega se lahko meritve površinske napetosti izvajajo v neravnovesnih stanjih, ko se velikost kapljice ali kemijska sestava okoliške tekočine s časom spreminjata. WGM elastometrija temelji na elastokapilarni interakciji med kapljicami in okoliško mehko matriko, pri čemer sta bila uporabljena hidrogel iz želatine in mišja tkiva. Matrika je bila nadzorovano deformirana, odziv kapljic pa merjen prek WGM-jev, kar je omogočilo določanje lokalnih anizotropnih napetosti, ki so lahko velikostnega reda le nekaj pN/μm^2. Poleg tega metoda omogoča meritev Youngovega modula matrike v območju 1kPa do 35kPa, kar zajema večino mehkih tkiv pri ljudeh. Razvite metode omogočajo natančnejšo določitev mehanskih lastnosti tekočin in mehkih materialov, kar je pomembno za širok spekter aplikacij, vključno s preučevanjem bioloških procesov.

• Keywords: mikrokapljice, ``whisperring gallery'' optične resonance, tekoče inkluzije, površinska napetost, elastokapilarnost, mehka snov, Youngov modul