Picometer resolved nanoscale optomechanics of micro-droplet

Abstract

Interaction of light with fluid produces many competing phenomena at the nanoscale, which are less well understood due to the lack of picometer precision in measuring optofluidic deformation. Here, we employ a microliter sessile fluid drop as a self-stabilized laser microinterferometer and resolve its nanoscale interface dynamics, with precisions of about 600 pm in real-time and 20 pm with a modulated beam, below the thermal limit. For evaporating droplets having various absorbance values, we isolate a nanodimple due to laser heating from the nanobump induced by Minkowski's optical momentum transfer. We model the dimple as resulting from a negative surface-tension thermal gradient induced by nonuniform local temperature variation, which we resolved with unprecedented 600 nK precision, besides detecting pN level radiation pressure force at the transparent fluid interface. These signatures are generic for a wide variety of fluids including Au-nanoparticle suspension, olive oil, glycerine, and biofluids such as egg-white and human saliva. Our study opens a route to achieve picometer precision with tiny fluid samples for intriguing applications.

Universal Stokes’s nanomechanical viscometer

“…Our detection technique offers sub-nanometer resolution to detect nano-mechanical waves with pm precision. Inset shows the experimental noise floor of which is calculated after subtracting the sinusoidal baseline 28 – 30 .

Figure 2 Propagation and dispersion of capillary waves on milk.

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“…Our technique is applicable for a wide variety of fluids including transparent, colloidal solutions, even highly absorbing black liquids having viscosity ranging from mPa s. Furthermore, we isolate additional damping caused by bottom friction for tiny droplets as well as issue of contamination of the sample in viscosity measurement. Enhanced damping significantly eliminated fluctuations in thin sessile liquid drops suggesting that such self stable drops, coupled with optical interferomters, form an attractive microscopic platform to study intriguing interface phenomenon with unprecedented picometer scale resolution 29 , 30 . Isolation of three distinct mechanisms of wave damping, especially differentiating bottom friction from internal viscous stresses opens a route to further miniaturize the setup to determine the viscosity with micro-liter fluid samples.…”

Contactless thin-film rheology unveiled by laser-induced nanoscale interface dynamics

“…Since Thus in presence of thermocapillarity with 0 T γ ∂ ∂ < , the induced flow should be radially directed away from the heating laser and due to mass conservation, the resulting toroidal flow should produce a beam-centred dimple on the top of the drop interface. [19][20][21] In the viscous regime, 58 ) to a position below the top of the non-deformed drop (negative height), demonstrating that a dimple has indeed been induced. Then this dimple relaxes much slower than expected from radiation pressure effects.…”

“…These requirements prompted the emergence of contactless micro-rheology techniques. Among all the various methods advanced, those based on the analysis of interface deformation have attracted attention for several important reasons: (i) most of the targeted properties of the liquid can be deduced from the analysis of both the dynamical and stationary deformations of the interface in different regimes (inertial, overdamped, …); (ii) Interfacial deformations can be triggered without any contact using several ways: from natural thermal fluctuations, 14 by capillary levelling, 13,15 by applying an electrostatic stress, 16,17,18 by triggering tangential thermal gradient and thermocapillary stresses, 19,20,21 or by irradiating the interface using the radiation pressure of an acoustic 22,23,24 or a laser beam. 25 , 26 The fluctuation method is based on the analysis of light scattering by broadband thermal fluctuations at the free surface and then it requires long signal acquisition durations.…”

“…shows that the deepness of this dimple is even more pronounced when decreasing the thickness h 0 . Then, in presence of a weak optical absorption, such a thermocapillary interface deformation should superimpose in opposite direction to the radiation pressure one21 when performing an experiment of optorheology. This is schematically illustrated in the Inset ofFig.…”