Abstract
Abstract Several fundamental research and applications in biomedicine and microfluidics often require controlled manipulation of suspended micro- and nanoscale particles. Speckle tweezers (ST) by incorporating randomly distributed light fields have been used to control micro-particles with refractive indices higher than their medium and to perform manipulation tasks such as guiding and sorting. Indeed, compared to periodic potentials, ST represents a wider possibility to be operated for such tasks. Here, we extend the usefulness of ST into micro-particles of low index with respect to the surrounding. Repelling of such particles by high intensity regions into lower intensity regions makes them to be locally confined, and the confinement can be tuned by changing the average grain intensity and size of the speckle patterns. Experiments on polystyrenes and liposomes validate the procedure. Moreover, we show that ST can also manipulate the nano-particle (NP)-loaded liposomes. Interestingly, the different interactions of NP-loaded and empty liposomes with ST enable collective manipulation of their mixture using the same speckle pattern, which may be explained by inclusion of the photophoretic forces on NPs. Our results on the different behaviors between empty and non-empty vesicles may open a new window on controlling collective transportation of drug micro-containers along with its wide applications in soft matter.
Highlights
Using light to control the motion of micro- and nanostructured objects is a challenge and involves several scientific and technological fields such as optical tweezing [1], Van der Waals and Casimir interactions [2, 3], integrated optics [4], biophysics [5], etc
The average intensity of the speckle grains is controlled by tuning the intensity of the laser light, and the grain size can be varied by careful adjustment of the fiber end to sample distance
The optical forces act on both high- and low-index particles stumbling in the speckle field in a characteristic time, the so-called waiting time, which is defined as the time that the particle moves along its correlation length with its average drift speed [43]
Summary
Using light to control the motion of micro- and nanostructured objects is a challenge and involves several scientific and technological fields such as optical tweezing [1], Van der Waals and Casimir interactions [2, 3], integrated optics [4], biophysics [5], etc. There is a specific field – optomechanics – in which the beneficial features of randomness have not been sufficiently investigated. Optomechanical forces come from the interaction of the electromagnetic wave with the boundaries of dielectric objects. Random systems include a large number of boundaries for which calculation of the forces is not trivial, but it is crucial to understand the mechanisms of optically activated devices [11]
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